Table of Contents
This chapter describes the syntax for the SQL statements supported by MySQL.
MySQL 8.0 supports atomic Data Definition Language (DDL) statements. This feature is referred to as atomic DDL. An atomic DDL statement combines the data dictionary updates, storage engine operations, and binary log writes associated with a DDL operation into a single, atomic transaction. The transaction is either committed, with applicable changes persisted to the data dictionary, storage engine, and binary log, or is rolled back, even if the server halts during the operation.
Atomic DDL is made possible by the introduction of the MySQL data dictionary in MySQL 8.0. In earlier MySQL versions, metadata was stored in metadata files, nontransactional tables, and storage engine-specific dictionaries, which necessitated intermediate commits. Centralized, transactional metadata storage provided by the MySQL data dictionary removed this barrier, making it possible to restructure DDL statement operations into atomic transactions.
The atomic DDL feature is described under the following topics in this section:
The atomic DDL feature supports both table and non-table DDL
statements. Table-related DDL operations require storage engine
support, whereas non-table DDL operations do not. Currently,
only the InnoDB
storage engine supports
atomic DDL.
Supported table DDL statements include
CREATE
, ALTER
, and
DROP
statements for databases,
tablespaces, tables, and indexes, and the
TRUNCATE TABLE
statement.
Supported non-table DDL statements include:
The following statements are not supported by the atomic DDL feature:
Table-related DDL statements that involve a storage engine
other than InnoDB
.
INSTALL PLUGIN
and
UNINSTALL PLUGIN
statements.
INSTALL COMPONENT
and
UNINSTALL COMPONENT
statements.
CREATE SERVER
,
ALTER SERVER
, and
DROP SERVER
statements.
The characteristics of atomic DDL statements include the following:
Metadata updates, binary log writes, and storage engine operations, where applicable, are combined into a single transaction.
There are no intermediate commits at the SQL layer during the DDL operation.
Where applicable:
The state of data dictionary, routine, event, and UDF caches is consistent with the status of the DDL operation, meaning that caches are updated to reflect whether or not the DDL operation was completed successfully or rolled back.
The storage engine methods involved in a DDL operation do not perform intermediate commits, and the storage engine registers itself as part of the DDL transaction.
The storage engine supports redo and rollback of DDL operations, which is performed in the Post-DDL phase of the DDL operation.
The visible behaviour of DDL operations is atomic, which changes the behavior of some DDL statements. See Changes in DDL Statement Behavior.
DDL statements, atomic or otherwise, implicitly end any
transaction that is active in the current session, as if you
had done a COMMIT
before
executing the statement. This means that DDL statements cannot
be performed within another transaction, within transaction
control statements such as
START TRANSACTION ...
COMMIT
, or combined with other statements within the
same transaction.
This section describes changes in DDL statement behavior due to the introduction of atomic DDL support.
DROP TABLE
operations are
fully atomic if all named tables use an atomic DDL-supported
storage engine. The statement either drops all tables
successfully or is rolled back.
DROP TABLE
fails with an
error if a named table does not exist, and no changes are
made, regardless of the storage engine. This change in
behavior is demonstrated in the following example, where the
DROP TABLE
statement fails
because a named table does not exist:
mysql>CREATE TABLE t1 (c1 INT);
mysql>DROP TABLE t1, t2;
ERROR 1051 (42S02): Unknown table 'test.t2' mysql>SHOW TABLES;
+----------------+ | Tables_in_test | +----------------+ | t1 | +----------------+
Prior to the introduction of atomic DDL,
DROP TABLE
reports an error
for the named table that does not exist but succeeds for the
named table that does exist:
mysql>CREATE TABLE t1 (c1 INT);
mysql>DROP TABLE t1, t2;
ERROR 1051 (42S02): Unknown table 'test.t2' mysql>SHOW TABLES;
Empty set (0.00 sec)
Due to this change in behavior, a partially completed
DROP TABLE
statement on a
MySQL 5.7 master fails when replicated on a MySQL 8.0
slave. To avoid this failure scenario, use IF
EXISTS
syntax in DROP
TABLE
statements to prevent errors from
occurring for tables that do not exist.
DROP DATABASE
is atomic if
all tables use an atomic DDL-supported storage engine. The
statement either drops all objects successfully or is rolled
back. However, removal of the database directory from the
file system occurs last and is not part of the atomic
transaction. If removal of the database directory fails due
to a file system error or server halt, the
DROP DATABASE
transaction is
not rolled back.
For tables that do not use an atomic DDL-supported storage
engine, table deletion occurs outside of the atomic
DROP TABLE
or
DROP DATABASE
transaction.
Such table deletions are written to the binary log
individually, which limits the discrepancy between the
storage engine, data dictionary, and binary log to one table
at most in the case of an interrupted
DROP TABLE
or
DROP DATABASE
operation. For
operations that drop multiple tables, the tables that do not
use an atomic DDL-supported storage engine are dropped
before tables that do.
CREATE TABLE
,
ALTER TABLE
,
RENAME TABLE
,
TRUNCATE TABLE
,
CREATE TABLESPACE
, and
DROP TABLESPACE
operations
for tables that use an atomic DDL-supported storage engine
are either fully committed or rolled back if the server
halts during their operation. In earlier MySQL releases,
interruption of these operations could cause discrepancies
between the storage engine, data dictionary, and binary log,
or leave behind orphan files. RENAME
TABLE
operations are only atomic if all named
tables use an atomic DDL-supported storage engine.
DROP VIEW
fails if a named
view does not exist, and no changes are made. The change in
behavior is demonstrated in this example, where the
DROP VIEW
statement fails
because a named view does not exist:
mysql>CREATE VIEW test.viewA AS SELECT * FROM t;
mysql>DROP VIEW test.viewA, test.viewB;
ERROR 1051 (42S02): Unknown table 'test.viewB' mysql>SHOW FULL TABLES IN test WHERE TABLE_TYPE LIKE 'VIEW';
+----------------+------------+ | Tables_in_test | Table_type | +----------------+------------+ | viewA | VIEW | +----------------+------------+
Prior to the introduction of atomic DDL,
DROP VIEW
returns an error
for the named view that does not exist but succeeds for the
named view that does exist:
mysql>CREATE VIEW test.viewA AS SELECT * FROM t;
mysql>DROP VIEW test.viewA, test.viewB;
ERROR 1051 (42S02): Unknown table 'test.viewB' mysql>SHOW FULL TABLES IN test WHERE TABLE_TYPE LIKE 'VIEW';
Empty set (0.00 sec)
Partial execution of account management statements is no longer permitted. Account management statements either succeed for all named users or roll back and have no effect if an error occurs. In earlier MySQL versions, account management statements that name multiple users could succeed for some users and fail for others.
The change in behavior is demonstrated in this example,
where the second CREATE USER
statement returns an error but fails because it cannot
succeed for all named users.
mysql>CREATE USER userA;
mysql>CREATE USER userA, userB;
ERROR 1396 (HY000): Operation CREATE USER failed for 'userA'@'%' mysql>SELECT User FROM mysql.user WHERE User LIKE 'user%';
+-------+ | User | +-------+ | userA | +-------+
Prior to the introduction of atomic DDL, the second
CREATE USER
statement returns an error
for the named user that does not exist but succeeds for the
named user that does exist:
mysql>CREATE USER userA;
mysql>CREATE USER userA, userB;
ERROR 1396 (HY000): Operation CREATE USER failed for 'userA'@'%' mysql>SELECT User FROM mysql.user WHERE User LIKE 'user%';
+-------+ | User | +-------+ | userA | | userB | +-------+
Due to this change in behavior, partially completed
account management statements on a MySQL 5.7 master fail
when replicated on a MySQL 8.0 slave. To avoid this
failure scenario, use IF EXISTS
or
IF NOT EXISTS
syntax, as appropriate,
in account management statements to prevent errors related
to named users.
Currently, only the InnoDB
storage engine
supports atomic DDL. Storage engines that do not support atomic
DDL are exempted from DDL atomicity. DDL operations involving
exempted storage engines remain capable of introducing
inconsistencies that can occur when operations are interrupted
or only partially completed.
To support redo and rollback of DDL operations,
InnoDB
writes DDL logs to the
mysql.innodb_ddl_log
table, which is a hidden
data dictionary table that resides in the
mysql.ibd
data dictionary tablespace.
To view DDL logs that are written to the
mysql.innodb_ddl_log
table during a DDL
operation, enable the
innodb_print_ddl_logs
configuration option. For more information, see
Viewing DDL Logs.
The redo logs for changes to the
mysql.innodb_ddl_log
table are flushed to
disk immediately regardless of the
innodb_flush_log_at_trx_commit
setting. Flushing the redo logs immediately avoids situations
where data files are modified by DDL operations but the redo
logs for changes to the
mysql.innodb_ddl_log
table resulting from
those operations are not persisted to disk. Such a situation
could cause errors during rollback or recovery.
The InnoDB
storage engine executes DDL
operations in phases. DDL operations such as
ALTER TABLE
may perform the
Prepare and Perform
phases multiple times prior to the Commit
phase.
Prepare: Create the required objects
and write the DDL logs to the
mysql.innodb_ddl_log
table. The DDL logs
define how to roll forward and roll back the DDL operation.
Perform: Perform the DDL operation. For
example, perform a create routine for a CREATE
TABLE
operation.
Commit: Update the data dictionary and commit the data dictionary transaction.
Post-DDL: Replay and remove DDL logs
from the mysql.innodb_ddl_log
table. To
ensure that rollback can be performed safely without
introducing inconsistencies, file operations such as
renaming or removing data files are performed in this final
phase. This phase also removes dynamic metadata from the
mysql.innodb_dynamic_metadata
data
dictionary table for DROP
TABLE
, TRUNCATE
TABLE
, and other DDL operations that rebuild the
table.
DDL logs are replayed and removed from the
mysql.innodb_ddl_log
table during the
Post-DDL phase, regardless of whether the
transaction is committed or rolled back. DDL logs should only
remain in the mysql.innodb_ddl_log
table if
the server is halted during a DDL operation. In this case, the
DDL logs are replayed and removed after recovery.
In a recovery situation, a DDL transaction may be committed or
rolled back when the server is restarted. If the data dictionary
transaction that was performed during the
Commit phase of a DDL operation is present
in the redo log and binary log, the operation is considered
successful and is rolled forward. Otherwise, the incomplete data
dictionary transaction is rolled back when
InnoDB
replays data dictionary redo logs, and
the DDL transaction is rolled back.
To view DDL logs that are written to the
mysql.innodb_ddl_log
data dictionary table
during atomic DDL operations that involve the
InnoDB
storage engine, enable
innodb_print_ddl_logs
to have
MySQL write the DDL logs to stderr
. Depending
on the host operating system and MySQL configuration,
stderr
may be the error log, terminal, or
console window. See
Section 5.4.2.2, “Default Error Log Destination Configuration”.
InnoDB
writes DDL logs to the
mysql.innodb_ddl_log
table to support redo
and rollback of DDL operations. The
mysql.innodb_ddl_log
table is a hidden data
dictionary table that resides in the
mysql.ibd
data dictionary tablespace. Like
other hidden data dictionary tables, the
mysql.innodb_ddl_log
table cannot be accessed
directly in non-debug versions of MySQL. (See
Section 14.1, “Data Dictionary Schema”.) The structure of the
mysql.innodb_ddl_log
table corresponds to
this definition:
CREATE TABLE mysql.innodb_ddl_log ( id BIGINT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY, thread_id BIGINT UNSIGNED NOT NULL, type INT UNSIGNED NOT NULL, space_id INT UNSIGNED, page_no INT UNSIGNED, index_id BIGINT UNSIGNED, table_id BIGINT UNSIGNED, old_file_path VARCHAR(512) COLLATE UTF8_BIN, new_file_path VARCHAR(512) COLLATE UTF8_BIN, KEY(thread_id) );
id
: A unique identifier for a DDL log
record.
thread_id
: Each DDL log record is
assigned a thread_id
, which is used to
replay and remove DDL logs that belong to a particular DDL
transaction. DDL transactions that involve multiple data
file operations generate multiple DDL log records.
type
: The DDL operation type. Types
include FREE
(drop an index tree),
DELETE
(delete a file),
RENAME
(rename a file), or
DROP
(drop metadata from the
mysql.innodb_dynamic_metadata
data
dictionary table).
space_id
: The tablespace ID.
page_no
: A page that contains allocation
information; an index tree root page, for example.
index_id
: The index ID.
table_id
: The table ID.
old_file_path
: The old tablespace file
path. Used by DDL operations that create or drop tablespace
files; also used by DDL operations that rename a tablespace.
new_file_path
: The new tablespace file
path. Used by DDL operations that rename tablespace files.
This example demonstrates enabling
innodb_print_ddl_logs
to view
DDL logs written to strderr
for a
CREATE TABLE
operation.
mysql> SET GLOBAL innodb_print_ddl_logs=1; mysql> CREATE TABLE t1 (c1 INT) ENGINE = InnoDB;
[Note] [000000] InnoDB: DDL log insert : [DDL record: DELETE SPACE, id=18, thread_id=7, space_id=5, old_file_path=./test/t1.ibd] [Note] [000000] InnoDB: DDL log delete : by id 18 [Note] [000000] InnoDB: DDL log insert : [DDL record: REMOVE CACHE, id=19, thread_id=7, table_id=1058, new_file_path=test/t1] [Note] [000000] InnoDB: DDL log delete : by id 19 [Note] [000000] InnoDB: DDL log insert : [DDL record: FREE, id=20, thread_id=7, space_id=5, index_id=132, page_no=4] [Note] [000000] InnoDB: DDL log delete : by id 20 [Note] [000000] InnoDB: DDL log post ddl : begin for thread id : 7 [Note] [000000] InnoDB: DDL log post ddl : end for thread id : 7
ALTER {DATABASE | SCHEMA} [db_name
]alter_specification
...alter_specification
: [DEFAULT] CHARACTER SET [=]charset_name
| [DEFAULT] COLLATE [=]collation_name
| DEFAULT ENCRYPTION [=] {'Y' | 'N'}
ALTER DATABASE
enables you to
change the overall characteristics of a database. These
characteristics are stored in the data dictionary. To use
ALTER DATABASE
, you need the
ALTER
privilege on the database.
ALTER
SCHEMA
is a synonym for ALTER
DATABASE
.
The database name can be omitted from the first syntax, in which case the statement applies to the default database.
The CHARACTER SET
clause changes the default
database character set. The COLLATE
clause
changes the default database collation. Chapter 10, Character Sets, Collations, Unicode,
discusses character set and collation names.
You can see what character sets and collations are available
using, respectively, the SHOW CHARACTER
SET
and SHOW COLLATION
statements. See Section 13.7.6.3, “SHOW CHARACTER SET Syntax”, and
Section 13.7.6.4, “SHOW COLLATION Syntax”, for more information.
If you change the default character set or collation for a database, stored routines that use the database defaults must be dropped and recreated so that they use the new defaults. (In a stored routine, variables with character data types use the database defaults if the character set or collation are not specified explicitly. See Section 13.1.17, “CREATE PROCEDURE and CREATE FUNCTION Syntax”.)
The DEFAULT ENCRYPTION
clause, introduced in
MySQL 8.0.16, defines the database encryption default that is
applied to all tables created in the database. An ALTER
DATABASE
statement executed without a DEFAULT
ENCRYPTION
clause retains the existing database
encryption default.
ALTER [DEFINER = {user
| CURRENT_USER }] EVENTevent_name
[ON SCHEDULEschedule
] [ON COMPLETION [NOT] PRESERVE] [RENAME TOnew_event_name
] [ENABLE | DISABLE | DISABLE ON SLAVE] [COMMENT 'string
'] [DOevent_body
]
The ALTER EVENT
statement changes
one or more of the characteristics of an existing event without
the need to drop and recreate it. The syntax for each of the
DEFINER
, ON SCHEDULE
,
ON COMPLETION
, COMMENT
,
ENABLE
/ DISABLE
, and
DO
clauses is exactly the same as
when used with CREATE EVENT
. (See
Section 13.1.13, “CREATE EVENT Syntax”.)
Any user can alter an event defined on a database for which that
user has the EVENT
privilege. When
a user executes a successful ALTER
EVENT
statement, that user becomes the definer for the
affected event.
ALTER EVENT
works only with an
existing event:
mysql>ALTER EVENT no_such_event
>ON SCHEDULE
>EVERY '2:3' DAY_HOUR;
ERROR 1517 (HY000): Unknown event 'no_such_event'
In each of the following examples, assume that the event named
myevent
is defined as shown here:
CREATE EVENT myevent ON SCHEDULE EVERY 6 HOUR COMMENT 'A sample comment.' DO UPDATE myschema.mytable SET mycol = mycol + 1;
The following statement changes the schedule for
myevent
from once every six hours starting
immediately to once every twelve hours, starting four hours from
the time the statement is run:
ALTER EVENT myevent ON SCHEDULE EVERY 12 HOUR STARTS CURRENT_TIMESTAMP + INTERVAL 4 HOUR;
It is possible to change multiple characteristics of an event in a
single statement. This example changes the SQL statement executed
by myevent
to one that deletes all records from
mytable
; it also changes the schedule for the
event such that it executes once, one day after this
ALTER EVENT
statement is run.
ALTER EVENT myevent ON SCHEDULE AT CURRENT_TIMESTAMP + INTERVAL 1 DAY DO TRUNCATE TABLE myschema.mytable;
Specify the options in an ALTER
EVENT
statement only for those characteristics that you
want to change; omitted options keep their existing values. This
includes any default values for CREATE
EVENT
such as ENABLE
.
To disable myevent
, use this
ALTER EVENT
statement:
ALTER EVENT myevent DISABLE;
The ON SCHEDULE
clause may use expressions
involving built-in MySQL functions and user variables to obtain
any of the timestamp
or
interval
values which it contains. You
cannot use stored routines or user-defined functions in such
expressions, and you cannot use any table references; however, you
can use SELECT FROM DUAL
. This is true for both
ALTER EVENT
and
CREATE EVENT
statements. References
to stored routines, user-defined functions, and tables in such
cases are specifically not permitted, and fail with an error (see
Bug #22830).
Although an ALTER EVENT
statement
that contains another ALTER EVENT
statement in its DO
clause appears
to succeed, when the server attempts to execute the resulting
scheduled event, the execution fails with an error.
To rename an event, use the ALTER
EVENT
statement's RENAME TO
clause.
This statement renames the event myevent
to
yourevent
:
ALTER EVENT myevent RENAME TO yourevent;
You can also move an event to a different database using
ALTER EVENT ... RENAME TO ...
and
notation, as shown here:
db_name.event_name
ALTER EVENT olddb.myevent RENAME TO newdb.myevent;
To execute the previous statement, the user executing it must have
the EVENT
privilege on both the
olddb
and newdb
databases.
There is no RENAME EVENT
statement.
The value DISABLE ON SLAVE
is used on a
replication slave instead of ENABLE
or
DISABLE
to indicate an event that was created
on the master and replicated to the slave, but that is not
executed on the slave. Normally, DISABLE ON
SLAVE
is set automatically as required; however, there
are some circumstances under which you may want or need to change
it manually. See Section 17.4.1.16, “Replication of Invoked Features”,
for more information.
ALTER FUNCTIONfunc_name
[characteristic
...]characteristic
: COMMENT 'string
' | LANGUAGE SQL | { CONTAINS SQL | NO SQL | READS SQL DATA | MODIFIES SQL DATA } | SQL SECURITY { DEFINER | INVOKER }
This statement can be used to change the characteristics of a
stored function. More than one change may be specified in an
ALTER FUNCTION
statement. However,
you cannot change the parameters or body of a stored function
using this statement; to make such changes, you must drop and
re-create the function using DROP
FUNCTION
and CREATE
FUNCTION
.
You must have the ALTER ROUTINE
privilege for the function. (That privilege is granted
automatically to the function creator.) If binary logging is
enabled, the ALTER FUNCTION
statement might also require the
SUPER
privilege, as described in
Section 24.7, “Binary Logging of Stored Programs”.
ALTER INSTANCEinstance_action
instance_action
: { ROTATE INNODB MASTER KEY | RELOAD TLS [NO ROLLBACK ON ERROR] }
ALTER INSTANCE
defines actions applicable to a
MySQL server instance. The statement supports these actions:
ALTER INSTANCE ROTATE INNODB MASTER KEY
This action rotates the master encryption key used for
InnoDB
tablespace encryption. Key rotation
requires the
ENCRYPTION_KEY_ADMIN
or
SUPER
privilege. To perform
this action, a keyring plugin must be installed and
configured. For instructions, see Section 6.5.4, “The MySQL Keyring”.
ALTER INSTANCE ROTATE INNODB MASTER KEY
supports concurrent DML. However, it cannot be run
concurrently with
CREATE TABLE ...
ENCRYPTION
or
ALTER TABLE ...
ENCRYPTION
operations, and locks are taken to
prevent conflicts that could arise from concurrent execution
of these statements. If one of the conflicting statements is
running, it must complete before another can proceed.
ALTER INSTANCE ROTATE INNODB MASTER KEY
statements are written to the binary log so that they can be
executed on replicated servers.
For additional ALTER INSTANCE ROTATE INNODB MASTER
KEY
usage information, see
Section 15.6.3.9, “Tablespace Encryption”.
This action reconfigures the SSL context from the current
values of the system variables that define the context. It
also updates the status variables that reflect the active
context values. This action requires the
CONNECTION_ADMIN
privilege.
By default, the RELOAD TLS
action rolls
back with an error and has no effect if the configuration
values do not permit creation of a new SSL context. The
previous context values continue to be used for new
connections.
If the optional NO ROLLBACK ON ERROR
clause
is given and a new context cannot be created, rollback does
not occur. Instead, a warning is generated and SSL is disabled
for new connections.
ALTER INSTANCE RELOAD TLS
statements are
not written to the binary log (and thus are not replicated).
SSL configuration is local and depends on local files not
necessarily present on all servers involved.
For additional information about reconfiguring the SSL context, including which system and status variables are context-related, see Server-Side Runtime Configuration for Encrypted Connections.
ALTER LOGFILE GROUPlogfile_group
ADD UNDOFILE 'file_name
' [INITIAL_SIZE [=]size
] [WAIT] ENGINE [=]engine_name
This statement adds an UNDO
file named
'file_name
' to an existing log file
group logfile_group
. An
ALTER LOGFILE GROUP
statement has
one and only one ADD UNDOFILE
clause. No
DROP UNDOFILE
clause is currently supported.
All NDB Cluster Disk Data objects share the same namespace. This means that each Disk Data object must be uniquely named (and not merely each Disk Data object of a given type). For example, you cannot have a tablespace and an undo log file with the same name, or an undo log file and a data file with the same name.
The optional INITIAL_SIZE
parameter sets the
UNDO
file's initial size in bytes; if not
specified, the initial size defaults to 134217728 (128 MB). You
may optionally follow size
with a
one-letter abbreviation for an order of magnitude, similar to
those used in my.cnf
. Generally, this is one
of the letters M
(megabytes) or
G
(gigabytes). (Bug #13116514, Bug #16104705,
Bug #62858)
On 32-bit systems, the maximum supported value for
INITIAL_SIZE
is 4294967296 (4 GB). (Bug #29186)
The minimum allowed value for INITIAL_SIZE
is
1048576 (1 MB). (Bug #29574)
WAIT
is parsed but otherwise ignored. This
keyword currently has no effect, and is intended for future
expansion.
The ENGINE
parameter (required) determines the
storage engine which is used by this log file group, with
engine_name
being the name of the
storage engine. Currently, the only accepted values for
engine_name
are
“NDBCLUSTER
” and
“NDB
”. The two values
are equivalent.
Here is an example, which assumes that the log file group
lg_3
has already been created using
CREATE LOGFILE GROUP
(see
Section 13.1.16, “CREATE LOGFILE GROUP Syntax”):
ALTER LOGFILE GROUP lg_3 ADD UNDOFILE 'undo_10.dat' INITIAL_SIZE=32M ENGINE=NDBCLUSTER;
When ALTER LOGFILE GROUP
is used
with ENGINE = NDBCLUSTER
(alternatively,
ENGINE = NDB
), an UNDO
log
file is created on each NDB Cluster data node. You can verify that
the UNDO
files were created and obtain
information about them by querying the
INFORMATION_SCHEMA.FILES
table. For
example:
mysql>SELECT FILE_NAME, LOGFILE_GROUP_NUMBER, EXTRA
->FROM INFORMATION_SCHEMA.FILES
->WHERE LOGFILE_GROUP_NAME = 'lg_3';
+-------------+----------------------+----------------+ | FILE_NAME | LOGFILE_GROUP_NUMBER | EXTRA | +-------------+----------------------+----------------+ | newdata.dat | 0 | CLUSTER_NODE=3 | | newdata.dat | 0 | CLUSTER_NODE=4 | | undo_10.dat | 11 | CLUSTER_NODE=3 | | undo_10.dat | 11 | CLUSTER_NODE=4 | +-------------+----------------------+----------------+ 4 rows in set (0.01 sec)
(See Section 25.11, “The INFORMATION_SCHEMA FILES Table”.)
Memory used for UNDO_BUFFER_SIZE
comes from the
global pool whose size is determined by the value of the
SharedGlobalMemory
data
node configuration parameter. This includes any default value
implied for this option by the setting of the
InitialLogFileGroup
data
node configuration parameter.
ALTER LOGFILE GROUP
is useful only
with Disk Data storage for NDB Cluster. For more information, see
Section 22.5.13, “NDB Cluster Disk Data Tables”.
ALTER PROCEDUREproc_name
[characteristic
...]characteristic
: COMMENT 'string
' | LANGUAGE SQL | { CONTAINS SQL | NO SQL | READS SQL DATA | MODIFIES SQL DATA } | SQL SECURITY { DEFINER | INVOKER }
This statement can be used to change the characteristics of a
stored procedure. More than one change may be specified in an
ALTER PROCEDURE
statement. However,
you cannot change the parameters or body of a stored procedure
using this statement; to make such changes, you must drop and
re-create the procedure using DROP
PROCEDURE
and CREATE
PROCEDURE
.
You must have the ALTER ROUTINE
privilege for the procedure. By default, that privilege is granted
automatically to the procedure creator. This behavior can be
changed by disabling the
automatic_sp_privileges
system
variable. See Section 24.2.2, “Stored Routines and MySQL Privileges”.
ALTER SERVERserver_name
OPTIONS (option
[,option
] ...)
Alters the server information for
,
adjusting any of the options permitted in the
server_name
CREATE SERVER
statement. The
corresponding fields in the mysql.servers
table
are updated accordingly. This statement requires the
SUPER
privilege.
For example, to update the USER
option:
ALTER SERVER s OPTIONS (USER 'sally');
ALTER SERVER
causes an implicit commit. See
Section 13.3.3, “Statements That Cause an Implicit Commit”.
ALTER SERVER
is not written to the binary log,
regardless of the logging format that is in use.
ALTER TABLEtbl_name
[alter_specification
[,alter_specification
] ...] [partition_options
]alter_specification
:table_options
| ADD [COLUMN]col_name
column_definition
[FIRST | AFTERcol_name
] | ADD [COLUMN] (col_name
column_definition
,...) | ADD {INDEX|KEY} [index_name
] [index_type
] (key_part
,...) [index_option
] ... | ADD {FULLTEXT|SPATIAL} [INDEX|KEY] [index_name
] (key_part
,...) [index_option
] ... | ADD [CONSTRAINT [symbol
]] PRIMARY KEY [index_type
] (key_part
,...) [index_option
] ... | ADD [CONSTRAINT [symbol
]] UNIQUE [INDEX|KEY] [index_name
] [index_type
] (key_part
,...) [index_option
] ... | ADD [CONSTRAINT [symbol
]] FOREIGN KEY [index_name
] (col_name
,...)reference_definition
| ADDcheck_constraint_definition
| DROP CHECKsymbol
| ALTER CHECKsymbol
[NOT] ENFORCED | ALGORITHM [=] {DEFAULT|INSTANT|INPLACE|COPY} | ALTER [COLUMN]col_name
{SET DEFAULTliteral
| DROP DEFAULT} | ALTER INDEXindex_name
{VISIBLE | INVISIBLE} | CHANGE [COLUMN]old_col_name
new_col_name
column_definition
[FIRST|AFTERcol_name
] | [DEFAULT] CHARACTER SET [=]charset_name
[COLLATE [=]collation_name
] | CONVERT TO CHARACTER SETcharset_name
[COLLATEcollation_name
] | {DISABLE|ENABLE} KEYS | {DISCARD|IMPORT} TABLESPACE | DROP [COLUMN]col_name
| DROP {INDEX|KEY}index_name
| DROP PRIMARY KEY | DROP FOREIGN KEYfk_symbol
| FORCE | LOCK [=] {DEFAULT|NONE|SHARED|EXCLUSIVE} | MODIFY [COLUMN]col_name
column_definition
[FIRST | AFTERcol_name
] | ORDER BYcol_name
[,col_name
] ... | RENAME COLUMNold_col_name
TOnew_col_name
| RENAME {INDEX|KEY}old_index_name
TOnew_index_name
| RENAME [TO|AS]new_tbl_name
| {WITHOUT|WITH} VALIDATION | ADD PARTITION (partition_definition
) | DROP PARTITIONpartition_names
| DISCARD PARTITION {partition_names
| ALL} TABLESPACE | IMPORT PARTITION {partition_names
| ALL} TABLESPACE | TRUNCATE PARTITION {partition_names
| ALL} | COALESCE PARTITIONnumber
| REORGANIZE PARTITIONpartition_names
INTO (partition_definitions
) | EXCHANGE PARTITIONpartition_name
WITH TABLEtbl_name
[{WITH|WITHOUT} VALIDATION] | ANALYZE PARTITION {partition_names
| ALL} | CHECK PARTITION {partition_names
| ALL} | OPTIMIZE PARTITION {partition_names
| ALL} | REBUILD PARTITION {partition_names
| ALL} | REPAIR PARTITION {partition_names
| ALL} | REMOVE PARTITIONINGkey_part
: {col_name
[(length
)] | (expr
)} [ASC | DESC]index_type
: USING {BTREE | HASH}index_option
: KEY_BLOCK_SIZE [=]value
|index_type
| WITH PARSERparser_name
| COMMENT 'string
' | {VISIBLE | INVISIBLE}check_constraint_definition
: [CONSTRAINT [symbol
]] CHECK (expr
) [[NOT] ENFORCED]table_options
:table_option
[[,]table_option
] ...table_option
: AUTO_INCREMENT [=]value
| AVG_ROW_LENGTH [=]value
| [DEFAULT] CHARACTER SET [=]charset_name
| CHECKSUM [=] {0 | 1} | [DEFAULT] COLLATE [=]collation_name
| COMMENT [=] 'string
' | COMPRESSION [=] {'ZLIB'|'LZ4'|'NONE'} | CONNECTION [=] 'connect_string
' | {DATA|INDEX} DIRECTORY [=] 'absolute path to directory
' | DELAY_KEY_WRITE [=] {0 | 1} | ENCRYPTION [=] {'Y' | 'N'} | ENGINE [=]engine_name
| INSERT_METHOD [=] { NO | FIRST | LAST } | KEY_BLOCK_SIZE [=]value
| MAX_ROWS [=]value
| MIN_ROWS [=]value
| PACK_KEYS [=] {0 | 1 | DEFAULT} | PASSWORD [=] 'string
' | ROW_FORMAT [=] {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT} | STATS_AUTO_RECALC [=] {DEFAULT|0|1} | STATS_PERSISTENT [=] {DEFAULT|0|1} | STATS_SAMPLE_PAGES [=]value
| TABLESPACEtablespace_name
[STORAGE {DISK|MEMORY}] | UNION [=] (tbl_name
[,tbl_name
]...)partition_options
: (seeCREATE TABLE
options)
ALTER TABLE
changes the structure
of a table. For example, you can add or delete columns, create or
destroy indexes, change the type of existing columns, or rename
columns or the table itself. You can also change characteristics
such as the storage engine used for the table or the table
comment.
To use ALTER TABLE
, you need
ALTER
,
CREATE
, and
INSERT
privileges for the
table. Renaming a table requires
ALTER
and
DROP
on the old table,
ALTER
,
CREATE
, and
INSERT
on the new table.
Following the table name, specify the alterations to be made.
If none are given, ALTER TABLE
does nothing.
The syntax for many of the permissible alterations is similar
to clauses of the CREATE TABLE
statement. column_definition
clauses use the same syntax for ADD
and
CHANGE
as for CREATE
TABLE
. For more information, see
Section 13.1.20, “CREATE TABLE Syntax”.
The word COLUMN
is optional and can be
omitted, except for RENAME COLUMN
(to
distinguish a column-renaming operation from the
RENAME
table-renaming operation).
Multiple ADD
, ALTER
,
DROP
, and CHANGE
clauses
are permitted in a single ALTER
TABLE
statement, separated by commas. This is a
MySQL extension to standard SQL, which permits only one of
each clause per ALTER TABLE
statement. For example, to drop multiple columns in a single
statement, do this:
ALTER TABLE t2 DROP COLUMN c, DROP COLUMN d;
If a storage engine does not support an attempted
ALTER TABLE
operation, a
warning may result. Such warnings can be displayed with
SHOW WARNINGS
. See
Section 13.7.6.40, “SHOW WARNINGS Syntax”. For information on
troubleshooting ALTER TABLE
,
see Section B.6.6.1, “Problems with ALTER TABLE”.
For information about generated columns, see Section 13.1.9.2, “ALTER TABLE and Generated Columns”.
For usage examples, see Section 13.1.9.3, “ALTER TABLE Examples”.
With the mysql_info()
C API
function, you can find out how many rows were copied by
ALTER TABLE
. See
Section 28.7.7.36, “mysql_info()”.
There are several additional aspects to the ALTER
TABLE
statement, described under the following topics in
this section:
table_options
signifies table options
of the kind that can be used in the CREATE
TABLE
statement, such as ENGINE
,
AUTO_INCREMENT
,
AVG_ROW_LENGTH
, MAX_ROWS
,
ROW_FORMAT
, or TABLESPACE
.
For descriptions of all table options, see
Section 13.1.20, “CREATE TABLE Syntax”. However,
ALTER TABLE
ignores DATA
DIRECTORY
and INDEX DIRECTORY
when
given as table options. ALTER TABLE
permits them only as partitioning options, and requires that you
have the FILE
privilege.
Use of table options with ALTER
TABLE
provides a convenient way of altering single table
characteristics. For example:
If t1
is currently not an
InnoDB
table, this statement changes its
storage engine to InnoDB
:
ALTER TABLE t1 ENGINE = InnoDB;
See Section 15.6.1.3, “Converting Tables from MyISAM to InnoDB” for
considerations when switching tables to the
InnoDB
storage engine.
When you specify an ENGINE
clause,
ALTER TABLE
rebuilds the
table. This is true even if the table already has the
specified storage engine.
Running ALTER
TABLE
on an existing
tbl_name
ENGINE=INNODBInnoDB
table performs a
“null” ALTER
TABLE
operation, which can be used to defragment
an InnoDB
table, as described in
Section 15.11.4, “Defragmenting a Table”. Running
ALTER TABLE
on an
tbl_name
FORCEInnoDB
table performs the same
function.
ALTER TABLE
and
tbl_name
ENGINE=INNODBALTER TABLE
use
online DDL. For
more information, see Section 15.12, “InnoDB and Online DDL”.
tbl_name
FORCE
The outcome of attempting to change the storage engine of
a table is affected by whether the desired storage engine
is available and the setting of the
NO_ENGINE_SUBSTITUTION
SQL mode, as described in Section 5.1.11, “Server SQL Modes”.
To prevent inadvertent loss of data,
ALTER TABLE
cannot be used
to change the storage engine of a table to
MERGE
or BLACKHOLE
.
To change the InnoDB
table to use
compressed row-storage format:
ALTER TABLE t1 ROW_FORMAT = COMPRESSED;
To enable or disable encryption for an
InnoDB
table in a file-per-table
tablespace:
ALTER TABLE t1 ENCRYPTION='Y'; ALTER TABLE t1 ENCRYPTION='N';
A keyring plugin must be installed and configured to use the
ENCRYPTION
option. For more information,
see Section 15.6.3.9, “Tablespace Encryption”.
To reset the current auto-increment value:
ALTER TABLE t1 AUTO_INCREMENT = 13;
You cannot reset the counter to a value less than or equal to
the value that is currently in use. For both
InnoDB
and MyISAM
, if
the value is less than or equal to the maximum value currently
in the AUTO_INCREMENT
column, the value is
reset to the current maximum AUTO_INCREMENT
column value plus one.
To change the default table character set:
ALTER TABLE t1 CHARACTER SET = utf8;
See also Changing the Character Set.
To add (or change) a table comment:
ALTER TABLE t1 COMMENT = 'New table comment';
Use ALTER TABLE
with the
TABLESPACE
option to move
InnoDB
tables between existing
general
tablespaces,
file-per-table
tablespaces, and the
system
tablespace. See
Moving Tables Between Tablespaces Using ALTER TABLE.
ALTER TABLE ... TABLESPACE
operations
always cause a full table rebuild, even if the
TABLESPACE
attribute has not changed
from its previous value.
ALTER TABLE ... TABLESPACE
syntax does
not support moving a table from a temporary tablespace to
a persistent tablespace.
The DATA DIRECTORY
clause, which is
supported with
CREATE TABLE
... TABLESPACE
, is not supported with
ALTER TABLE ... TABLESPACE
, and is
ignored if specified.
For more information about the capabilities and
limitations of the TABLESPACE
option,
see CREATE TABLE
.
MySQL NDB Cluster 8.0 supports setting
NDB_TABLE
options for controlling a
table's partition balance (fragment count type),
read-from-any-replica capability, full replication, or any
combination of these, as part of the table comment for an
ALTER TABLE
statement in the same manner as
for CREATE TABLE
, as shown in
this example:
ALTER TABLE t1 COMMENT = "NDB_TABLE=READ_BACKUP=0,PARTITION_BALANCE=FOR_RA_BY_NODE";
Bear in mind that ALTER TABLE ... COMMENT
...
discards any existing comment for the table. See
Setting NDB_TABLE options, for
additional information and examples.
To verify that the table options were changed as intended, use
SHOW CREATE TABLE
, or query the
INFORMATION_SCHEMA.TABLES
table.
ALTER TABLE
operations are
processed using one of the following algorithms:
COPY
: Operations are performed on a copy of
the original table, and table data is copied from the original
table to the new table row by row. Concurrent DML is not
permitted.
INPLACE
: Operations avoid copying table
data but may rebuild the table in place. An exclusive metadata
lock on the table may be taken briefly during preparation and
execution phases of the operation. Typically, concurrent DML
is supported.
INSTANT
: Operations only modify metadata in
the data dictionary. No exclusive metadata locks are taken on
the table during preparation and execution, and table data is
unaffected, making operations instantaneous. Concurrent DML is
permitted. (Introduced in MySQL 8.0.12)
The ALGORITHM
clause is optional. If the
ALGORITHM
clause is omitted, MySQL uses
ALGORITHM=INSTANT
for storage engines and
ALTER TABLE
clauses that support
it. Otherwise, ALGORITHM=INPLACE
is used. If
ALGORITHM=INPLACE
is not supported,
ALGORITHM=COPY
is used.
Specifying an ALGORITHM
clause requires the
operation to use the specified algorithm for clauses and storage
engines that support it, or fail with an error otherwise.
Specifying ALGORITHM=DEFAULT
is the same as
omitting the ALGORITHM
clause.
ALTER TABLE
operations that use the
COPY
algorithm wait for other operations that
are modifying the table to complete. After alterations are applied
to the table copy, data is copied over, the original table is
deleted, and the table copy is renamed to the name of the original
table. While the ALTER TABLE
operation executes, the original table is readable by other
sessions (with the exception noted shortly). Updates and writes to
the table started after the ALTER
TABLE
operation begins are stalled until the new table
is ready, then are automatically redirected to the new table. The
temporary copy of the table is created in the database directory
of the original table unless it is a RENAME TO
operation that moves the table to a database that resides in a
different directory.
The exception referred to earlier is that
ALTER TABLE
blocks reads (not just
writes) at the point where it is ready to clear outdated table
structures from the table and table definition caches. At this
point, it must acquire an exclusive lock. To do so, it waits for
current readers to finish, and blocks new reads and writes.
An ALTER TABLE
operation that uses
the COPY
algorithm prevents concurrent DML
operations. Concurrent queries are still allowed. That is, a
table-copying operation always includes at least the concurrency
restrictions of LOCK=SHARED
(allow queries but
not DML). You can further restrict concurrency for operations that
support the LOCK
clause by specifying
LOCK=EXCLUSIVE
, which prevents DML and queries.
For more information, see
Concurrency Control.
To force use of the COPY
algorithm for an
ALTER TABLE
operation that would
otherwise not use it, specify ALGORITHM=COPY
or
enable the old_alter_table
system
variable. If there is a conflict between the
old_alter_table
setting and an
ALGORITHM
clause with a value other than
DEFAULT
, the ALGORITHM
clause takes precedence.
For InnoDB
tables, an
ALTER TABLE
operation that uses the
COPY
algorithm on a table that resides in a
shared tablespace
can increase the amount of space used by the tablespace. Such
operations require as much additional space as the data in the
table plus indexes. For a table residing in a shared tablespace,
the additional space used during the operation is not released
back to the operating system as it is for a table that resides in
a file-per-table
tablespace.
For information about space requirements for online DDL operations, see Section 15.12.3, “Online DDL Space Requirements”.
ALTER TABLE
operations that support
the INPLACE
algorithm include:
ALTER TABLE
operations supported by the
InnoDB
online DDL feature. See
Section 15.12.1, “Online DDL Operations”.
Renaming a table. MySQL renames files that correspond to the
table tbl_name
without making a
copy. (You can also use the RENAME
TABLE
statement to rename tables. See
Section 13.1.36, “RENAME TABLE Syntax”.) Privileges granted
specifically for the renamed table are not migrated to the new
name. They must be changed manually.
Operations that only modify table metadata. These operations are immediate because the server does not touch table contents. Metadata-only operations include:
Renaming a column.
Changing the default value of a column (except for
NDB
tables).
Modifying the definition of an
ENUM
or
SET
column by adding new
enumeration or set members to the end
of the list of valid member values, as long as the storage
size of the data type does not change. For example, adding
a member to a SET
column
that has 8 members changes the required storage per value
from 1 byte to 2 bytes; this requires a table copy. Adding
members in the middle of the list causes renumbering of
existing members, which requires a table copy.
Changing the definition of a spatial column to remove the
SRID
attribute. (Adding or changing an
SRID
attribute does require a rebuild
and cannot be done in place because the server must verify
that all values have the specified SRID value.)
As of MySQL 8.0.14, changing a column character set, when these conditions apply:
As of MySQL 8.0.14, changing a generated column, when these conditions apply:
For InnoDB
tables, statements that
modify generated stored columns but do not change
their type, expression, or nullability.
For non-InnoDB
tables, statements
that modify generated stored or virtual columns but do
not change their type, expression, or nullability.
An example of such a change is a change to the column comment.
Renaming an index.
Adding or dropping a secondary index, for
InnoDB
and
NDB
tables. See
Section 15.12.1, “Online DDL Operations”.
For NDB
tables, operations that
add and drop indexes on variable-width columns. These
operations occur online, without table copying and without
blocking concurrent DML actions for most of their duration.
See Section 22.5.14, “Online Operations with ALTER TABLE in NDB Cluster”.
Modifying index visibility with an ALTER
INDEX
operation.
Column modifications of tables containing generated columns
that depend on columns with a DEFAULT
value
if the modified columns are not involved in the generated
column expressions. For example, changing the
NULL
property of a separate column can be
done in place without a table rebuild.
ALTER TABLE
operations that support the
INSTANT
algorithm include:
Adding a column. This feature is referred to as “Instant
ADD COLUMN
”. Limitations apply. See
Section 15.12.1, “Online DDL Operations”.
Adding or dropping a virtual column.
Adding or dropping a column default value.
Modifying the definition of an
ENUM
or
SET
column. The same
restrictions apply as described above for
ALGORITHM=INSTANT
.
Changing the index type.
Renaming a table. The same restrictions apply as described
above for ALGORITHM=INSTANT
.
For more information about operations that support
ALGORITHM=INSTANT
, see
Section 15.12.1, “Online DDL Operations”.
ALTER TABLE
upgrades MySQL 5.5
temporal columns to 5.6 format for ADD COLUMN
,
CHANGE COLUMN
, MODIFY
COLUMN
, ADD INDEX
, and
FORCE
operations. This conversion cannot be
done using the INPLACE
algorithm because the
table must be rebuilt, so specifying
ALGORITHM=INPLACE
in these cases results in an
error. Specify ALGORITHM=COPY
if necessary.
If an ALTER TABLE
operation on a multicolumn
index used to partition a table by KEY
changes
the order of the columns, it can only be performed using
ALGORITHM=COPY
.
The WITHOUT VALIDATION
and WITH
VALIDATION
clauses affect whether
ALTER TABLE
performs an in-place
operation for
virtual generated
column modifications. See
Section 13.1.9.2, “ALTER TABLE and Generated Columns”.
NDB Cluster 8.0 supports online operations using the same
ALGORITHM=INPLACE
syntax used with the standard
MySQL Server. See
Section 22.5.14, “Online Operations with ALTER TABLE in NDB Cluster”, for more
information.
ALTER TABLE
with DISCARD ... PARTITION
... TABLESPACE
or IMPORT ... PARTITION ...
TABLESPACE
does not create any temporary tables or
temporary partition files.
ALTER TABLE
with ADD
PARTITION
, DROP PARTITION
,
COALESCE PARTITION
, REBUILD
PARTITION
, or REORGANIZE PARTITION
does not create temporary tables (except when used with
NDB
tables); however, these
operations can and do create temporary partition files.
ADD
or DROP
operations for
RANGE
or LIST
partitions are
immediate operations or nearly so. ADD
or
COALESCE
operations for HASH
or KEY
partitions copy data between all
partitions, unless LINEAR HASH
or
LINEAR KEY
was used; this is effectively the
same as creating a new table, although the ADD
or COALESCE
operation is performed partition by
partition. REORGANIZE
operations copy only
changed partitions and do not touch unchanged ones.
For MyISAM
tables, you can speed up index
re-creation (the slowest part of the alteration process) by
setting the
myisam_sort_buffer_size
system
variable to a high value.
For ALTER TABLE
operations that
support it, you can use the LOCK
clause to
control the level of concurrent reads and writes on a table while
it is being altered. Specifying a non-default value for this
clause enables you to require a certain amount of concurrent
access or exclusivity during the alter operation, and halts the
operation if the requested degree of locking is not available.
Only LOCK = DEFAULT
is permitted for operations
that use ALGORITHM=INSTANT
. The other
LOCK
clause parameters are not applicable.
The parameters for the LOCK
clause are:
LOCK = DEFAULT
Maximum level of concurrency for the given
ALGORITHM
clause (if any) and
ALTER TABLE
operation: Permit concurrent
reads and writes if supported. If not, permit concurrent reads
if supported. If not, enforce exclusive access.
LOCK = NONE
If supported, permit concurrent reads and writes. Otherwise, an error occurs.
LOCK = SHARED
If supported, permit concurrent reads but block writes. Writes
are blocked even if concurrent writes are supported by the
storage engine for the given ALGORITHM
clause (if any) and ALTER TABLE
operation.
If concurrent reads are not supported, an error occurs.
LOCK = EXCLUSIVE
Enforce exclusive access. This is done even if concurrent
reads/writes are supported by the storage engine for the given
ALGORITHM
clause (if any) and
ALTER TABLE
operation.
Use ADD
to add new columns to a table, and
DROP
to remove existing columns. DROP
is a MySQL extension
to standard SQL.
col_name
To add a column at a specific position within a table row, use
FIRST
or AFTER
. The default is to
add the column last.
col_name
If a table contains only one column, the column cannot be dropped.
If what you intend is to remove the table, use the
DROP TABLE
statement instead.
If columns are dropped from a table, the columns are also removed
from any index of which they are a part. If all columns that make
up an index are dropped, the index is dropped as well. If you use
CHANGE
or MODIFY
to shorten
a column for which an index exists on the column, and the
resulting column length is less than the index length, MySQL
shortens the index automatically.
For ALTER TABLE ... ADD
, if the column has an
expression default value that uses a nondeterministic function,
the statement may produce a warning or error. For details, see
Section 17.1.3.6, “Restrictions on Replication with GTIDs”.
The CHANGE
, MODIFY
,
RENAME COLUMN
, and ALTER
clauses enable the names and definitions of existing columns to be
altered. They have these comparative characteristics:
CHANGE
:
Can rename a column and change its definition, or both.
Has more capability than MODIFY
or
RENAME COLUMN
, but at the expense of
convenience for some operations. CHANGE
requires naming the column twice if not renaming it, and
requires respecifying the column definition if only
renaming it.
With FIRST
or AFTER
,
can reorder columns.
MODIFY
:
Can change a column definition but not its name.
More convenient than CHANGE
to change a
column definition without renaming it.
With FIRST
or AFTER
,
can reorder columns.
RENAME COLUMN
:
Can change a column name but not its definition.
More convenient than CHANGE
to rename a
column without changing its definition.
ALTER
: Used only to change a column default
value.
CHANGE
is a MySQL extension to standard SQL.
MODIFY
and RENAME COLUMN
are
MySQL extensions for Oracle compatibility.
To alter a column to change both its name and definition, use
CHANGE
, specifying the old and new names and
the new definition. For example, to rename an INT NOT
NULL
column from a
to
b
and change its definition to use the
BIGINT
data type while retaining the
NOT NULL
attribute, do this:
ALTER TABLE t1 CHANGE a b BIGINT NOT NULL;
To change a column definition but not its name, use
CHANGE
or MODIFY
. With
CHANGE
, the syntax requires two column names,
so you must specify the same name twice to leave the name
unchanged. For example, to change the definition of column
b
, do this:
ALTER TABLE t1 CHANGE b b INT NOT NULL;
MODIFY
is more convenient to change the
definition without changing the name because it requires the
column name only once:
ALTER TABLE t1 MODIFY b INT NOT NULL;
To change a column name but not its definition, use
CHANGE
or RENAME COLUMN
.
With CHANGE
, the syntax requires a column
definition, so to leave the definition unchanged, you must
respecify the definition the column currently has. For example, to
rename an INT NOT NULL
column from
b
to a
, do this:
ALTER TABLE t1 CHANGE b a INT NOT NULL;
RENAME COLUMN
is more convenient to change the
name without changing the definition because it requires only the
old and new names:
ALTER TABLE t1 RENAME COLUMN b TO a;
In general, you cannot rename a column to a name that already
exists in the table. However, this is sometimes not the case, such
as when you swap names or move them through a cycle. If a table
has columns named a
, b
, and
c
, these are valid operations:
-- swap a and b ALTER TABLE t1 RENAME COLUMN a TO b, RENAME COLUMN b TO a; -- "rotate" a, b, c through a cycle ALTER TABLE t1 RENAME COLUMN a TO b, RENAME COLUMN b TO c, RENAME COLUMN c TO a;
For column definition changes using CHANGE
or
MODIFY
, the definition must include the data
type and all attributes that should apply to the new column, other
than index attributes such as PRIMARY KEY
or
UNIQUE
. Attributes present in the original
definition but not specified for the new definition are not
carried forward. Suppose that a column col1
is
defined as INT UNSIGNED DEFAULT 1 COMMENT 'my
column'
and you modify the column as follows, intending
to change only INT
to
BIGINT
:
ALTER TABLE t1 MODIFY col1 BIGINT;
That statement changes the data type from INT
to BIGINT
, but it also drops the
UNSIGNED
, DEFAULT
, and
COMMENT
attributes. To retain them, the
statement must include them explicitly:
ALTER TABLE t1 MODIFY col1 BIGINT UNSIGNED DEFAULT 1 COMMENT 'my column';
For data type changes using CHANGE
or
MODIFY
, MySQL tries to convert existing column
values to the new type as well as possible.
This conversion may result in alteration of data. For example,
if you shorten a string column, values may be truncated. To
prevent the operation from succeeding if conversions to the new
data type would result in loss of data, enable strict SQL mode
before using ALTER TABLE
(see
Section 5.1.11, “Server SQL Modes”).
If you use CHANGE
or MODIFY
to shorten a column for which an index exists on the column, and
the resulting column length is less than the index length, MySQL
shortens the index automatically.
For columns renamed by CHANGE
or
RENAME COLUMN
, MySQL automatically renames
these references to the renamed column:
Indexes that refer to the old column, including invisible
indexes and disabled MyISAM
indexes.
Foreign keys that refer to the old column.
For columns renamed by CHANGE
or
RENAME COLUMN
, MySQL does not automatically
rename these references to the renamed column:
Generated column and partition expressions that refer to the
renamed column. You must use CHANGE
to
redefine such expressions in the same
ALTER TABLE
statement as the
one that renames the column.
Views and stored programs that refer to the renamed column. You must manually alter the definition of these objects to refer to the new column name.
To reorder columns within a table, use FIRST
and AFTER
in CHANGE
or
MODIFY
operations.
ALTER ... SET DEFAULT
or ALTER ...
DROP DEFAULT
specify a new default value for a column or
remove the old default value, respectively. If the old default is
removed and the column can be NULL
, the new
default is NULL
. If the column cannot be
NULL
, MySQL assigns a default value as
described in Section 11.7, “Data Type Default Values”.
DROP PRIMARY KEY
drops the
primary key. If there is
no primary key, an error occurs. For information about the
performance characteristics of primary keys, especially for
InnoDB
tables, see
Section 8.3.2, “Primary Key Optimization”.
If you add a UNIQUE INDEX
or PRIMARY
KEY
to a table, MySQL stores it before any nonunique
index to permit detection of duplicate keys as early as possible.
DROP INDEX
removes an index. This
is a MySQL extension to standard SQL. See
Section 13.1.27, “DROP INDEX Syntax”. To determine index names, use
SHOW INDEX FROM
.
tbl_name
Some storage engines permit you to specify an index type when
creating an index. The syntax for the
index_type
specifier is USING
. For details about
type_name
USING
, see Section 13.1.15, “CREATE INDEX Syntax”. The
preferred position is after the column list. Support for use of
the option before the column list will be removed in a future
MySQL release.
index_option
values specify additional
options for an index. USING
is one such option.
For details about permissible
index_option
values, see
Section 13.1.15, “CREATE INDEX Syntax”.
RENAME INDEX
renames an
index. This is a MySQL extension to standard SQL. The content of
the table remains unchanged.
old_index_name
TO
new_index_name
old_index_name
must be the name of an
existing index in the table that is not dropped by the same
ALTER TABLE
statement.
new_index_name
is the new index name,
which cannot duplicate the name of an index in the resulting table
after changes have been applied. Neither index name can be
PRIMARY
.
If you use ALTER TABLE
on a
MyISAM
table, all nonunique indexes are created
in a separate batch (as for REPAIR
TABLE
). This should make ALTER
TABLE
much faster when you have many indexes.
For MyISAM
tables, key updating can be
controlled explicitly. Use ALTER TABLE ... DISABLE
KEYS
to tell MySQL to stop updating nonunique indexes.
Then use ALTER TABLE ... ENABLE KEYS
to
re-create missing indexes. MyISAM
does this
with a special algorithm that is much faster than inserting keys
one by one, so disabling keys before performing bulk insert
operations should give a considerable speedup. Using
ALTER TABLE ... DISABLE KEYS
requires the
INDEX
privilege in addition to the
privileges mentioned earlier.
While the nonunique indexes are disabled, they are ignored for
statements such as SELECT
and
EXPLAIN
that otherwise would use
them.
After an ALTER TABLE
statement, it
may be necessary to run ANALYZE
TABLE
to update index cardinality information. See
Section 13.7.6.22, “SHOW INDEX Syntax”.
The ALTER INDEX
operation permits an index to
be made visible or invisible. An invisible index is not used by
the optimizer. Modification of index visibility applies to indexes
other than primary keys (either explicit or implicit). This
feature is storage engine neutral (supported for any engine). For
more information, see Section 8.3.12, “Invisible Indexes”.
The FOREIGN KEY
and
REFERENCES
clauses are supported by the
InnoDB
and NDB
storage
engines, which implement ADD [CONSTRAINT
[
. See
Section 15.6.1.5, “InnoDB and FOREIGN KEY Constraints”. For other
storage engines, the clauses are parsed but ignored.
symbol
]] FOREIGN KEY
[index_name
] (...) REFERENCES ...
(...)
For ALTER TABLE
, unlike
CREATE TABLE
, ADD FOREIGN
KEY
ignores index_name
if
given and uses an automatically generated foreign key name. As a
workaround, include the CONSTRAINT
clause to
specify the foreign key name:
ADD CONSTRAINT name
FOREIGN KEY (....) ...
MySQL silently ignores inline REFERENCES
specifications, where the references are defined as part of the
column specification. MySQL accepts only
REFERENCES
clauses defined as part of a
separate FOREIGN KEY
specification.
Partitioned InnoDB
tables do not support
foreign keys. This restriction does not apply to
NDB
tables, including those explicitly
partitioned by [LINEAR] KEY
. For more
information, see
Section 23.6.2, “Partitioning Limitations Relating to Storage Engines”.
MySQL Server and NDB Cluster both support the use of
ALTER TABLE
to drop foreign keys:
ALTER TABLEtbl_name
DROP FOREIGN KEYfk_symbol
;
Adding and dropping a foreign key in the same
ALTER TABLE
statement is supported
for ALTER TABLE ...
ALGORITHM=INPLACE
but not for
ALTER TABLE ...
ALGORITHM=COPY
.
The server prohibits changes to foreign key columns that have the
potential to cause loss of referential integrity. It also
prohibits changes to the data type of such columns that may be
unsafe. For example, changing
VARCHAR(20)
to
VARCHAR(30)
is permitted, but
changing it to VARCHAR(1024)
is not
because that alters the number of length bytes required to store
individual values. A workaround is to use
ALTER TABLE ... DROP
FOREIGN KEY
before changing the column definition and
ALTER TABLE ... ADD
FOREIGN KEY
afterward.
ALTER TABLE
changes
internally generated foreign key constraint names and user-defined
foreign key constraint names that begin with the string
“tbl_name
RENAME
new_tbl_name
tbl_name
_ibfk_” to
reflect the new table name. InnoDB
interprets
foreign key constraint names that begin with the string
“tbl_name
_ibfk_” as
internally generated names.
Prior to MySQL 8.0.16, ALTER TABLE
permits only the following limited version of
CHECK
constraint-adding syntax, which is parsed
and ignored:
ADD CHECK (expr
)
As of MySQL 8.0.16, ALTER TABLE
permits CHECK
constraints for existing tables
to be added, dropped, or altered:
Add a new constraint:
ALTER TABLEtbl_name
ADD CONSTRAINT [symbol
] CHECK (expr
) [[NOT] ENFORCED];
The meaning of constraint syntax elements is the same as for
CREATE TABLE
. See
Section 13.1.20.7, “CHECK Constraints”.
Drop an existing constraint named
symbol
:
ALTER TABLEtbl_name
DROP CHECKsymbol
;
Alter whether an existing constraint named
symbol
is enforced:
ALTER TABLEtbl_name
ALTER CHECKsymbol
[NOT] ENFORCED;
The DROP CHECK
and ALTER
CHECK
clauses are MySQL extensions to standard SQL.
If a table alteration causes a violation of an enforced
CHECK
constraint, an error occurs and the table
is not modified. Examples of operations for which an error occurs:
Attempts to add the AUTO_INCREMENT
attribute to a column that is used in a
CHECK
constraint.
Attempts to add an enforced CHECK
constraint or enforce a nonenforced CHECK
constraint for which existing rows violate the constraint
condition.
Attempts to modify, rename, or drop a column that is used in a
CHECK
constraint, unless that constraint is
also dropped in the same statement. Exception: If a
CHECK
constraint refers only to a single
column, dropping the column automatically drops the
constraint.
ALTER TABLE
changes
internally generated and user-defined tbl_name
RENAME
new_tbl_name
CHECK
constraint names that begin with the string
“tbl_name
_chk_” to reflect
the new table name. MySQL interprets CHECK
constraint names that begin with the string
“tbl_name
_chk_” as
internally generated names.
To change the table default character set and all character
columns (CHAR
,
VARCHAR
,
TEXT
) to a new character set, use a
statement like this:
ALTER TABLEtbl_name
CONVERT TO CHARACTER SETcharset_name
;
The statement also changes the collation of all character columns.
If you specify no COLLATE
clause to indicate
which collation to use, the statement uses default collation for
the character set. If this collation is inappropriate for the
intended table use (for example, if it would change from a
case-sensitive collation to a case-insensitive collation), specify
a collation explicitly.
For a column that has a data type of
VARCHAR
or one of the
TEXT
types, CONVERT TO
CHARACTER SET
changes the data type as necessary to
ensure that the new column is long enough to store as many
characters as the original column. For example, a
TEXT
column has two length bytes,
which store the byte-length of values in the column, up to a
maximum of 65,535. For a latin1
TEXT
column, each character
requires a single byte, so the column can store up to 65,535
characters. If the column is converted to utf8
,
each character might require up to three bytes, for a maximum
possible length of 3 × 65,535 = 196,605 bytes. That length
does not fit in a TEXT
column's
length bytes, so MySQL converts the data type to
MEDIUMTEXT
, which is the smallest
string type for which the length bytes can record a value of
196,605. Similarly, a VARCHAR
column might be converted to
MEDIUMTEXT
.
To avoid data type changes of the type just described, do not use
CONVERT TO CHARACTER SET
. Instead, use
MODIFY
to change individual columns. For
example:
ALTER TABLE t MODIFY latin1_text_col TEXT CHARACTER SET utf8;
ALTER TABLE t MODIFY latin1_varchar_col VARCHAR(M
) CHARACTER SET utf8;
If you specify CONVERT TO CHARACTER SET binary
,
the CHAR
,
VARCHAR
, and
TEXT
columns are converted to their
corresponding binary string types
(BINARY
,
VARBINARY
,
BLOB
). This means that the columns
no longer will have a character set and a subsequent
CONVERT TO
operation will not apply to them.
If charset_name
is
DEFAULT
in a CONVERT TO CHARACTER
SET
operation, the character set named by the
character_set_database
system
variable is used.
The CONVERT TO
operation converts column
values between the original and named character sets. This is
not what you want if you have a column in
one character set (like latin1
) but the
stored values actually use some other, incompatible character
set (like utf8
). In this case, you have to do
the following for each such column:
ALTER TABLE t1 CHANGE c1 c1 BLOB; ALTER TABLE t1 CHANGE c1 c1 TEXT CHARACTER SET utf8;
The reason this works is that there is no conversion when you
convert to or from BLOB
columns.
To change only the default character set for a table, use this statement:
ALTER TABLEtbl_name
DEFAULT CHARACTER SETcharset_name
;
The word DEFAULT
is optional. The default
character set is the character set that is used if you do not
specify the character set for columns that you add to a table
later (for example, with ALTER TABLE ... ADD
column
).
When the foreign_key_checks
system variable is enabled, which is the default setting,
character set conversion is not permitted on tables that include a
character string column used in a foreign key constraint. The
workaround is to disable
foreign_key_checks
before
performing the character set conversion. You must perform the
conversion on both tables involved in the foreign key constraint
before re-enabling
foreign_key_checks
. If you
re-enable foreign_key_checks
after converting only one of the tables, an ON DELETE
CASCADE
or ON UPDATE CASCADE
operation could corrupt data in the referencing table due to
implicit conversion that occurs during these operations (Bug
#45290, Bug #74816).
An InnoDB
table created in its own
file-per-table
tablespace can be discarded and imported using the
DISCARD TABLESPACE
and IMPORT
TABLESPACE
options. These options can be used to import
a file-per-table tablespace from a backup or to copy a
file-per-table tablespace from one database server to another. See
Section 15.6.3.7, “Copying Tablespaces to Another Instance”.
ORDER BY
enables you to create the new table
with the rows in a specific order. This option is useful primarily
when you know that you query the rows in a certain order most of
the time. By using this option after major changes to the table,
you might be able to get higher performance. In some cases, it
might make sorting easier for MySQL if the table is in order by
the column that you want to order it by later.
The table does not remain in the specified order after inserts and deletes.
ORDER BY
syntax permits one or more column
names to be specified for sorting, each of which optionally can be
followed by ASC
or DESC
to
indicate ascending or descending sort order, respectively. The
default is ascending order. Only column names are permitted as
sort criteria; arbitrary expressions are not permitted. This
clause should be given last after any other clauses.
ORDER BY
does not make sense for
InnoDB
tables because InnoDB
always orders table rows according to the
clustered index.
When used on a partitioned table, ALTER TABLE ... ORDER
BY
orders rows within each partition only.
partition_options
signifies options
that can be used with partitioned tables for repartitioning, to
add, drop, discard, import, merge, and split partitions, and to
perform partitioning maintenance.
It is possible for an ALTER TABLE
statement to contain a PARTITION BY
or
REMOVE PARTITIONING
clause in an addition to
other alter specifications, but the PARTITION
BY
or REMOVE PARTITIONING
clause must
be specified last after any other specifications. The ADD
PARTITION
, DROP PARTITION
,
DISCARD PARTITION
, IMPORT
PARTITION
, COALESCE PARTITION
,
REORGANIZE PARTITION
, EXCHANGE
PARTITION
, ANALYZE PARTITION
,
CHECK PARTITION
, and REPAIR
PARTITION
options cannot be combined with other alter
specifications in a single ALTER TABLE
, since
the options just listed act on individual partitions.
For more information about partition options, see
Section 13.1.20, “CREATE TABLE Syntax”, and
Section 13.1.9.1, “ALTER TABLE Partition Operations”. For
information about and examples of ALTER TABLE ...
EXCHANGE PARTITION
statements, see
Section 23.3.3, “Exchanging Partitions and Subpartitions with Tables”.
Partitioning-related clauses for ALTER
TABLE
can be used with partitioned tables for
repartitioning, to add, drop, discard, import, merge, and split
partitions, and to perform partitioning maintenance.
Simply using a partition_options
clause with ALTER TABLE
on a
partitioned table repartitions the table according to the
partitioning scheme defined by the
partition_options
. This clause
always begins with PARTITION BY
, and
follows the same syntax and other rules as apply to the
partition_options
clause for
CREATE TABLE
(for more
detailed information, see Section 13.1.20, “CREATE TABLE Syntax”),
and can also be used to partition an existing table that is
not already partitioned. For example, consider a
(nonpartitioned) table defined as shown here:
CREATE TABLE t1 ( id INT, year_col INT );
This table can be partitioned by HASH
,
using the id
column as the partitioning
key, into 8 partitions by means of this statement:
ALTER TABLE t1 PARTITION BY HASH(id) PARTITIONS 8;
MySQL supports an ALGORITHM
option with
[SUB]PARTITION BY [LINEAR] KEY
.
ALGORITHM=1
causes the server to use the
same key-hashing functions as MySQL 5.1 when computing the
placement of rows in partitions;
ALGORITHM=2
means that the server employs
the key-hashing functions implemented and used by default
for new KEY
partitioned tables in MySQL
5.5 and later. (Partitioned tables created with the
key-hashing functions employed in MySQL 5.5 and later cannot
be used by a MySQL 5.1 server.) Not specifying the option
has the same effect as using ALGORITHM=2
.
This option is intended for use chiefly when upgrading or
downgrading [LINEAR] KEY
partitioned
tables between MySQL 5.1 and later MySQL versions, or for
creating tables partitioned by KEY
or
LINEAR KEY
on a MySQL 5.5 or later server
which can be used on a MySQL 5.1 server.
The table that results from using an ALTER TABLE
... PARTITION BY
statement must follow the same
rules as one created using CREATE TABLE ...
PARTITION BY
. This includes the rules governing
the relationship between any unique keys (including any
primary key) that the table might have, and the column or
columns used in the partitioning expression, as discussed in
Section 23.6.1, “Partitioning Keys, Primary Keys, and Unique Keys”.
The CREATE TABLE ... PARTITION BY
rules
for specifying the number of partitions also apply to
ALTER TABLE ... PARTITION BY
.
The partition_definition
clause
for ALTER TABLE ADD PARTITION
supports
the same options as the clause of the same name for the
CREATE TABLE
statement. (See
Section 13.1.20, “CREATE TABLE Syntax”, for the syntax and
description.) Suppose that you have the partitioned table
created as shown here:
CREATE TABLE t1 ( id INT, year_col INT ) PARTITION BY RANGE (year_col) ( PARTITION p0 VALUES LESS THAN (1991), PARTITION p1 VALUES LESS THAN (1995), PARTITION p2 VALUES LESS THAN (1999) );
You can add a new partition p3
to this
table for storing values less than 2002
as follows:
ALTER TABLE t1 ADD PARTITION (PARTITION p3 VALUES LESS THAN (2002));
DROP PARTITION
can be used to drop one or
more RANGE
or LIST
partitions. This statement cannot be used with
HASH
or KEY
partitions; instead, use COALESCE
PARTITION
(see later in this section). Any data
that was stored in the dropped partitions named in the
partition_names
list is
discarded. For example, given the table
t1
defined previously, you can drop the
partitions named p0
and
p1
as shown here:
ALTER TABLE t1 DROP PARTITION p0, p1;
DROP PARTITION
does not work with
tables that use the NDB
storage engine. See
Section 23.3.1, “Management of RANGE and LIST Partitions”, and
Section 22.1.7, “Known Limitations of NDB Cluster”.
ADD PARTITION
and DROP
PARTITION
do not currently support IF
[NOT] EXISTS
.
The DISCARD
PARTITION ... TABLESPACE
and
IMPORT
PARTITION ... TABLESPACE
options extend the
Transportable
Tablespace feature to individual
InnoDB
table partitions. Each
InnoDB
table partition has its own
tablespace file (.idb
file). The
Transportable
Tablespace feature makes it easy to copy the
tablespaces from a running MySQL server instance to another
running instance, or to perform a restore on the same
instance. Both options take a comma-separated list of one or
more partition names. For example:
ALTER TABLE t1 DISCARD PARTITION p2, p3 TABLESPACE;
ALTER TABLE t1 IMPORT PARTITION p2, p3 TABLESPACE;
When running
DISCARD
PARTITION ... TABLESPACE
and
IMPORT
PARTITION ... TABLESPACE
on subpartitioned tables,
both partition and subpartition names are allowed. When a
partition name is specified, subpartitions of that partition
are included.
The
Transportable
Tablespace feature also supports copying or restoring
partitioned InnoDB
tables (all partitions
at once). For additional information, see
Section 15.6.3.7, “Copying Tablespaces to Another Instance”, as well as,
Section 15.6.3.7.1, “Transportable Tablespace Examples”.
Renames of partitioned tables are supported. You can rename
individual partitions indirectly using ALTER TABLE
... REORGANIZE PARTITION
; however, this operation
copies the partition's data.
To delete rows from selected partitions, use the
TRUNCATE PARTITION
option. This option
takes a list of one or more comma-separated partition names.
Consider the table t1
created by this
statement:
CREATE TABLE t1 ( id INT, year_col INT ) PARTITION BY RANGE (year_col) ( PARTITION p0 VALUES LESS THAN (1991), PARTITION p1 VALUES LESS THAN (1995), PARTITION p2 VALUES LESS THAN (1999), PARTITION p3 VALUES LESS THAN (2003), PARTITION p4 VALUES LESS THAN (2007) );
To delete all rows from partition p0
, use
the following statement:
ALTER TABLE t1 TRUNCATE PARTITION p0;
The statement just shown has the same effect as the
following DELETE
statement:
DELETE FROM t1 WHERE year_col < 1991;
When truncating multiple partitions, the partitions do not
have to be contiguous: This can greatly simplify delete
operations on partitioned tables that would otherwise
require very complex WHERE
conditions if
done with DELETE
statements.
For example, this statement deletes all rows from partitions
p1
and p3
:
ALTER TABLE t1 TRUNCATE PARTITION p1, p3;
An equivalent DELETE
statement is shown here:
DELETE FROM t1 WHERE (year_col >= 1991 AND year_col < 1995) OR (year_col >= 2003 AND year_col < 2007);
If you use the ALL
keyword in place of
the list of partition names, the statement acts on all table
partitions.
TRUNCATE PARTITION
merely deletes rows;
it does not alter the definition of the table itself, or of
any of its partitions.
To verify that the rows were dropped, check the
INFORMATION_SCHEMA.PARTITIONS
table,
using a query such as this one:
SELECT PARTITION_NAME, TABLE_ROWS FROM INFORMATION_SCHEMA.PARTITIONS WHERE TABLE_NAME = 't1';
COALESCE PARTITION
can be used with a
table that is partitioned by HASH
or
KEY
to reduce the number of partitions by
number
. Suppose that you have
created table t2
as follows:
CREATE TABLE t2 ( name VARCHAR (30), started DATE ) PARTITION BY HASH( YEAR(started) ) PARTITIONS 6;
To reduce the number of partitions used by
t2
from 6 to 4, use the following
statement:
ALTER TABLE t2 COALESCE PARTITION 2;
The data contained in the last
number
partitions will be merged
into the remaining partitions. In this case, partitions 4
and 5 will be merged into the first 4 partitions (the
partitions numbered 0, 1, 2, and 3).
To change some but not all the partitions used by a
partitioned table, you can use REORGANIZE
PARTITION
. This statement can be used in several
ways:
To merge a set of partitions into a single partition.
This is done by naming several partitions in the
partition_names
list and
supplying a single definition for
partition_definition
.
To split an existing partition into several partitions.
Accomplish this by naming a single partition for
partition_names
and providing
multiple
partition_definitions
.
To change the ranges for a subset of partitions defined
using VALUES LESS THAN
or the value
lists for a subset of partitions defined using
VALUES IN
.
For partitions that have not been explicitly named, MySQL
automatically provides the default names
p0
, p1
,
p2
, and so on. The same is true with
regard to subpartitions.
For more detailed information about and examples of
ALTER TABLE ... REORGANIZE PARTITION
statements, see
Section 23.3.1, “Management of RANGE and LIST Partitions”.
To exchange a table partition or subpartition with a table,
use the ALTER
TABLE ... EXCHANGE PARTITION
statement—that
is, to move any existing rows in the partition or
subpartition to the nonpartitioned table, and any existing
rows in the nonpartitioned table to the table partition or
subpartition.
For usage information and examples, see Section 23.3.3, “Exchanging Partitions and Subpartitions with Tables”.
Several options provide partition maintenance and repair
functionality analogous to that implemented for
nonpartitioned tables by statements such as
CHECK TABLE
and
REPAIR TABLE
(which are also
supported for partitioned tables; for more information, see
Section 13.7.3, “Table Maintenance Statements”). These include
ANALYZE PARTITION
, CHECK
PARTITION
, OPTIMIZE PARTITION
,
REBUILD PARTITION
, and REPAIR
PARTITION
. Each of these options takes a
partition_names
clause consisting
of one or more names of partitions, separated by commas. The
partitions must already exist in the target table. You can
also use the ALL
keyword in place of
partition_names
, in which case
the statement acts on all table partitions. For more
information and examples, see
Section 23.3.4, “Maintenance of Partitions”.
InnoDB
does not currently
support per-partition optimization; ALTER TABLE ...
OPTIMIZE PARTITION
causes the entire table to
rebuilt and analyzed, and an appropriate warning to be
issued. (Bug #11751825, Bug #42822) To work around this
problem, use ALTER TABLE ... REBUILD
PARTITION
and ALTER TABLE ... ANALYZE
PARTITION
instead.
The ANALYZE PARTITION
, CHECK
PARTITION
, OPTIMIZE PARTITION
,
and REPAIR PARTITION
options are not
supported for tables which are not partitioned.
REMOVE PARTITIONING
enables you to remove
a table's partitioning without otherwise affecting the
table or its data. This option can be combined with other
ALTER TABLE
options such as
those used to add, drop, or rename columns or indexes.
Using the ENGINE
option with
ALTER TABLE
changes the
storage engine used by the table without affecting the
partitioning. The target storage engine must provide its own
partitioning handler. Only the InnoDB
and
NDB
storage engines have native
partitioning handlers; NDB
is not
currently supported in MySQL 8.0.
It is possible for an ALTER TABLE
statement to contain a PARTITION BY
or
REMOVE PARTITIONING
clause in an addition to
other alter specifications, but the PARTITION
BY
or REMOVE PARTITIONING
clause
must be specified last after any other specifications.
The ADD PARTITION
, DROP
PARTITION
, COALESCE PARTITION
,
REORGANIZE PARTITION
, ANALYZE
PARTITION
, CHECK PARTITION
, and
REPAIR PARTITION
options cannot be combined
with other alter specifications in a single ALTER
TABLE
, since the options just listed act on individual
partitions. For more information, see
Section 13.1.9.1, “ALTER TABLE Partition Operations”.
Only a single instance of any one of the following options can
be used in a given ALTER TABLE
statement: PARTITION BY
, ADD
PARTITION
, DROP PARTITION
,
TRUNCATE PARTITION
, EXCHANGE
PARTITION
, REORGANIZE PARTITION
, or
COALESCE PARTITION
, ANALYZE
PARTITION
, CHECK PARTITION
,
OPTIMIZE PARTITION
, REBUILD
PARTITION
, REMOVE PARTITIONING
.
For example, the following two statements are invalid:
ALTER TABLE t1 ANALYZE PARTITION p1, ANALYZE PARTITION p2; ALTER TABLE t1 ANALYZE PARTITION p1, CHECK PARTITION p2;
In the first case, you can analyze partitions
p1
and p2
of table
t1
concurrently using a single statement with
a single ANALYZE PARTITION
option that lists
both of the partitions to be analyzed, like this:
ALTER TABLE t1 ANALYZE PARTITION p1, p2;
In the second case, it is not possible to perform
ANALYZE
and CHECK
operations on different partitions of the same table
concurrently. Instead, you must issue two separate statements,
like this:
ALTER TABLE t1 ANALYZE PARTITION p1; ALTER TABLE t1 CHECK PARTITION p2;
REBUILD
operations are currently unsupported
for subpartitions. The REBUILD
keyword is
expressly disallowed with subpartitions, and causes
ALTER TABLE
to fail with an error if so used.
CHECK PARTITION
and REPAIR
PARTITION
operations fail when the partition to be
checked or repaired contains any duplicate key errors.
For more information about these statements, see Section 23.3.4, “Maintenance of Partitions”.
ALTER TABLE
operations permitted for
generated columns are ADD
,
MODIFY
, and CHANGE
.
Generated columns can be added.
CREATE TABLE t1 (c1 INT); ALTER TABLE t1 ADD COLUMN c2 INT GENERATED ALWAYS AS (c1 + 1) STORED;
The data type and expression of generated columns can be modified.
CREATE TABLE t1 (c1 INT, c2 INT GENERATED ALWAYS AS (c1 + 1) STORED); ALTER TABLE t1 MODIFY COLUMN c2 TINYINT GENERATED ALWAYS AS (c1 + 5) STORED;
Generated columns can be renamed or dropped, if no other column refers to them.
CREATE TABLE t1 (c1 INT, c2 INT GENERATED ALWAYS AS (c1 + 1) STORED); ALTER TABLE t1 CHANGE c2 c3 INT GENERATED ALWAYS AS (c1 + 1) STORED; ALTER TABLE t1 DROP COLUMN c3;
Virtual generated columns cannot be altered to stored generated columns, or vice versa. To work around this, drop the column, then add it with the new definition.
CREATE TABLE t1 (c1 INT, c2 INT GENERATED ALWAYS AS (c1 + 1) VIRTUAL); ALTER TABLE t1 DROP COLUMN c2; ALTER TABLE t1 ADD COLUMN c2 INT GENERATED ALWAYS AS (c1 + 1) STORED;
Nongenerated columns can be altered to stored but not virtual generated columns.
CREATE TABLE t1 (c1 INT, c2 INT); ALTER TABLE t1 MODIFY COLUMN c2 INT GENERATED ALWAYS AS (c1 + 1) STORED;
Stored but not virtual generated columns can be altered to nongenerated columns. The stored generated values become the values of the nongenerated column.
CREATE TABLE t1 (c1 INT, c2 INT GENERATED ALWAYS AS (c1 + 1) STORED); ALTER TABLE t1 MODIFY COLUMN c2 INT;
ADD COLUMN
is not an in-place operation
for stored columns (done without using a temporary table)
because the expression must be evaluated by the server. For
stored columns, indexing changes are done in place, and
expression changes are not done in place. Changes to column
comments are done in place.
For non-partitioned tables, ADD COLUMN
and DROP COLUMN
are in-place operations
for virtual columns. However, adding or dropping a virtual
column cannot be performed in place in combination with
other ALTER TABLE
operations.
For partitioned tables, ADD COLUMN
and
DROP COLUMN
are not in-place operations
for virtual columns.
InnoDB
supports secondary indexes on
virtual generated columns. Adding or dropping a secondary
index on a virtual generated column is an in-place
operation. For more information, see
Section 13.1.20.10, “Secondary Indexes and Generated Columns”.
When a VIRTUAL
generated column is added
to a table or modified, it is not ensured that data being
calculated by the generated column expression will not be
out of range for the column. This can lead to inconsistent
data being returned and unexpectedly failed statements. To
permit control over whether validation occurs for such
columns, ALTER TABLE
supports
WITHOUT VALIDATION
and WITH
VALIDATION
clauses:
With WITHOUT VALIDATION
(the default
if neither clause is specified), an in-place operation
is performed (if possible), data integrity is not
checked, and the statement finishes more quickly.
However, later reads from the table might report
warnings or errors for the column if values are out of
range.
With WITH VALIDATION
, ALTER
TABLE
copies the table. If an out-of-range or
any other error occurs, the statement fails. Because a
table copy is performed, the statement takes longer.
WITHOUT VALIDATION
and WITH
VALIDATION
are permitted only with ADD
COLUMN
, CHANGE COLUMN
, and
MODIFY COLUMN
operations. Otherwise, an
ER_WRONG_USAGE
error occurs.
If expression evaluation causes truncation or provides
incorrect input to a function, the
ALTER TABLE
statement
terminates with an error and the DDL operation is rejected.
An ALTER TABLE
statement that
changes the default value of a column
col_name
may also change the
value of a generated column expression that refers to the
column using col_name
, which may
change the value of a generated column expression that
refers to the column using
DEFAULT(
.
For this reason, col_name
)ALTER TABLE
operations that change the definition of a column cause a
table rebuild if any generated column expression uses
DEFAULT()
.
Begin with a table t1
created as shown here:
CREATE TABLE t1 (a INTEGER, b CHAR(10));
To rename the table from t1
to
t2
:
ALTER TABLE t1 RENAME t2;
To change column a
from
INTEGER
to TINYINT NOT
NULL
(leaving the name the same), and to change column
b
from CHAR(10)
to
CHAR(20)
as well as renaming it from
b
to c
:
ALTER TABLE t2 MODIFY a TINYINT NOT NULL, CHANGE b c CHAR(20);
To add a new TIMESTAMP
column
named d
:
ALTER TABLE t2 ADD d TIMESTAMP;
To add an index on column d
and a
UNIQUE
index on column a
:
ALTER TABLE t2 ADD INDEX (d), ADD UNIQUE (a);
To remove column c
:
ALTER TABLE t2 DROP COLUMN c;
To add a new AUTO_INCREMENT
integer column
named c
:
ALTER TABLE t2 ADD c INT UNSIGNED NOT NULL AUTO_INCREMENT, ADD PRIMARY KEY (c);
We indexed c
(as a PRIMARY
KEY
) because AUTO_INCREMENT
columns
must be indexed, and we declare c
as
NOT NULL
because primary key columns cannot
be NULL
.
For NDB
tables, it is also possible
to change the storage type used for a table or column. For
example, consider an NDB
table
created as shown here:
mysql> CREATE TABLE t1 (c1 INT) TABLESPACE ts_1 ENGINE NDB;
Query OK, 0 rows affected (1.27 sec)
To convert this table to disk-based storage, you can use the
following ALTER TABLE
statement:
mysql>ALTER TABLE t1 TABLESPACE ts_1 STORAGE DISK;
Query OK, 0 rows affected (2.99 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql>SHOW CREATE TABLE t1\G
*************************** 1. row *************************** Table: t1 Create Table: CREATE TABLE `t1` ( `c1` int(11) DEFAULT NULL ) /*!50100 TABLESPACE ts_1 STORAGE DISK */ ENGINE=ndbcluster DEFAULT CHARSET=latin1 1 row in set (0.01 sec)
It is not necessary that the tablespace was referenced when the
table was originally created; however, the tablespace must be
referenced by the ALTER TABLE
:
mysql>CREATE TABLE t2 (c1 INT) ts_1 ENGINE NDB;
Query OK, 0 rows affected (1.00 sec) mysql>ALTER TABLE t2 STORAGE DISK;
ERROR 1005 (HY000): Can't create table 'c.#sql-1750_3' (errno: 140) mysql>ALTER TABLE t2 TABLESPACE ts_1 STORAGE DISK;
Query OK, 0 rows affected (3.42 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql>SHOW CREATE TABLE t2\G
*************************** 1. row *************************** Table: t1 Create Table: CREATE TABLE `t2` ( `c1` int(11) DEFAULT NULL ) /*!50100 TABLESPACE ts_1 STORAGE DISK */ ENGINE=ndbcluster DEFAULT CHARSET=latin1 1 row in set (0.01 sec)
To change the storage type of an individual column, you can use
ALTER TABLE ... MODIFY [COLUMN]
. For example,
suppose you create an NDB Cluster Disk Data table with two
columns, using this CREATE TABLE
statement:
mysql>CREATE TABLE t3 (c1 INT, c2 INT)
->TABLESPACE ts_1 STORAGE DISK ENGINE NDB;
Query OK, 0 rows affected (1.34 sec)
To change column c2
from disk-based to
in-memory storage, include a STORAGE MEMORY clause in the column
definition used by the ALTER TABLE statement, as shown here:
mysql> ALTER TABLE t3 MODIFY c2 INT STORAGE MEMORY;
Query OK, 0 rows affected (3.14 sec)
Records: 0 Duplicates: 0 Warnings: 0
You can make an in-memory column into a disk-based column by
using STORAGE DISK
in a similar fashion.
Column c1
uses disk-based storage, since this
is the default for the table (determined by the table-level
STORAGE DISK
clause in the
CREATE TABLE
statement). However,
column c2
uses in-memory storage, as can be
seen here in the output of SHOW CREATE
TABLE
:
mysql> SHOW CREATE TABLE t3\G
*************************** 1. row ***************************
Table: t3
Create Table: CREATE TABLE `t3` (
`c1` int(11) DEFAULT NULL,
`c2` int(11) /*!50120 STORAGE MEMORY */ DEFAULT NULL
) /*!50100 TABLESPACE ts_1 STORAGE DISK */ ENGINE=ndbcluster DEFAULT CHARSET=latin1
1 row in set (0.02 sec)
When you add an AUTO_INCREMENT
column, column
values are filled in with sequence numbers automatically. For
MyISAM
tables, you can set the first sequence
number by executing SET
INSERT_ID=
before
value
ALTER TABLE
or by using the
AUTO_INCREMENT=
table option.
value
With MyISAM
tables, if you do not change the
AUTO_INCREMENT
column, the sequence number is
not affected. If you drop an AUTO_INCREMENT
column and then add another AUTO_INCREMENT
column, the numbers are resequenced beginning with 1.
When replication is used, adding an
AUTO_INCREMENT
column to a table might not
produce the same ordering of the rows on the slave and the
master. This occurs because the order in which the rows are
numbered depends on the specific storage engine used for the
table and the order in which the rows were inserted. If it is
important to have the same order on the master and slave, the
rows must be ordered before assigning an
AUTO_INCREMENT
number. Assuming that you want
to add an AUTO_INCREMENT
column to the table
t1
, the following statements produce a new
table t2
identical to t1
but with an AUTO_INCREMENT
column:
CREATE TABLE t2 (id INT AUTO_INCREMENT PRIMARY KEY) SELECT * FROM t1 ORDER BY col1, col2;
This assumes that the table t1
has columns
col1
and col2
.
This set of statements will also produce a new table
t2
identical to t1
, with
the addition of an AUTO_INCREMENT
column:
CREATE TABLE t2 LIKE t1; ALTER TABLE t2 ADD id INT AUTO_INCREMENT PRIMARY KEY; INSERT INTO t2 SELECT * FROM t1 ORDER BY col1, col2;
To guarantee the same ordering on both master and slave,
all columns of t1
must
be referenced in the ORDER BY
clause.
Regardless of the method used to create and populate the copy
having the AUTO_INCREMENT
column, the final
step is to drop the original table and then rename the copy:
DROP TABLE t1; ALTER TABLE t2 RENAME t1;
ALTER [UNDO] TABLESPACEtablespace_name
NDB only: {ADD|DROP} DATAFILE 'file_name
' [INITIAL_SIZE [=] size] [WAIT] InnoDB and NDB: [RENAME TOtablespace_name
] InnoDB only: [SET {ACTIVE|INACTIVE}] [ENCRYPTION [=] {'Y' | 'N'}] InnoDB and NDB: [ENGINE [=]engine_name
]
This statement is used with NDB
and
InnoDB
tablespaces. It can be used to add a new
data file to, or to drop a data file from an
NDB
tablespace. It can also be used to rename
an NDB Cluster Disk Data tablespace, rename an
InnoDB
general tablespace, encrypt an
InnoDB
general tablespace, or mark an
InnoDB
undo tablespace as active or inactive.
The UNDO
keyword, introduced in MySQL 8.0.14,
is used with the SET {ACTIVE|INACTIVE}
clause
to mark an InnoDB
undo tablespace as active or
inactive. For more information, see
Section 15.6.3.4, “Undo Tablespaces”.
The ADD DATAFILE
variant enables you to specify
an initial size for an NDB
Disk Data tablespace
using an INITIAL_SIZE
clause, where
size
is measured in bytes; the default
value is 134217728 (128 MB). You may optionally follow
size
with a one-letter abbreviation for
an order of magnitude, similar to those used in
my.cnf
. Generally, this is one of the letters
M
(megabytes) or G
(gigabytes).
On 32-bit systems, the maximum supported value for
INITIAL_SIZE
is 4294967296 (4 GB). (Bug #29186)
INITIAL_SIZE
is rounded, explicitly, as for
CREATE TABLESPACE
.
Once a data file has been created, its size cannot be changed;
however, you can add more data files to an NDB tablespace using
additional ALTER TABLESPACE ... ADD DATAFILE
statements.
When ALTER TABLESPACE ... ADD DATAFILE
is used
with ENGINE = NDB
, a data file is created on
each Cluster data node, but only one row is generated in the
INFORMATION_SCHEMA.FILES
table. See
the description of this table, as well as
Section 22.5.13.1, “NDB Cluster Disk Data Objects”, for more
information. ADD DATAFILE
is not supported with
InnoDB
tablespaces.
Using DROP DATAFILE
with
ALTER TABLESPACE
drops the data
file 'file_name
' from an NDB
tablespace. You cannot drop a data file from a tablespace which is
in use by any table; in other words, the data file must be empty
(no extents used). See
Section 22.5.13.1, “NDB Cluster Disk Data Objects”. In addition,
any data file to be dropped must previously have been added to the
tablespace with CREATE TABLESPACE
or ALTER TABLESPACE
. DROP
DATAFILE
is not supported with InnoDB
tablespaces.
WAIT
is parsed but otherwise ignored. It is
intended for future expansion.
The ENGINE
clause, which specifies the storage
engine used by the tablespace, is deprecated and will be removed
in a future release. The tablespace storage engine is known by the
data dictionary, making the ENGINE
clause
obsolete. If the storage engine is specified, it must match the
tablespace storage engine defined in the data dictionary. The only
values for engine_name
compatible with
NDB
tablespaces are
NDB
and NDBCLUSTER
.
RENAME TO
operations are implicitly performed
in autocommit
mode, regardless of
the autocommit
setting.
A RENAME TO
operation cannot be performed while
LOCK TABLES
or
FLUSH TABLES WITH READ
LOCK
is in effect for tables that reside in the
tablespace.
Exclusive metadata locks are taken on tables that reside in a general tablespace while the tablespace is renamed, which prevents concurrent DDL. Concurrent DML is supported.
The CREATE TABLESPACE
privilege is
required to rename an InnoDB
general
tablespace.
The ENCRYPTION
option enables or disables
page-level data encryption for an InnoDB
general tablespace or the mysql
system
tablespace. Option values are not case-sensitive. Encryption
support for general tablespaces was introduced in MySQL 8.0.13.
Encryption support for the mysql
system
tablespace was introduced in MySQL 8.0.16.
If the
table_encryption_privilege_check
variable is enabled, the
TABLE_ENCRYPTION_ADMIN
privilege is
required to alter a tablespace with an
ENCRYPTION
option that differs from the
default_table_encryption
setting.
The
table_encryption_privilege_check
and default_table_encryption
variables were introduced in MySQL 8.0.16.
Enabling encryption fails if any table in the tablespace belongs
to a schema defined with
DEFAULT
ENCRYPTION='N'
. Similarly, disabling encryption fails if
any table in the tablespace belongs to a schema defined with
DEFAULT
ENCRYPTION='Y'
. The
DEFAULT
ENCRYPTION
schema option was introduced in MySQL 8.0.16.
If an ALTER TABLESPACE
statement
does not include an ENCRYPTION
option, the
tablespace retains its current encryption state, regardless of the
default_table_encryption
setting.
A keyring plugin must be installed and configured before encryption can be enabled.
When a general tablespace or the mysql
system
tablespace is encrypted, all tables residing in the tablespace are
encrypted. Likewise, a table created in an encrypted tablespace is
encrypted.
The INPLACE
algorithm is used when altering the
ENCRYPTION
attribute of a general tablespace or
the mysql
system tablespace. The
INPLACE
algorithm permits concurrent DML on
tables that reside in the tablespace. Concurrent DDL is blocked.
For more information, see Section 15.6.3.9, “Tablespace Encryption”.
ALTER [ALGORITHM = {UNDEFINED | MERGE | TEMPTABLE}] [DEFINER = {user
| CURRENT_USER }] [SQL SECURITY { DEFINER | INVOKER }] VIEWview_name
[(column_list
)] ASselect_statement
[WITH [CASCADED | LOCAL] CHECK OPTION]
This statement changes the definition of a view, which must exist.
The syntax is similar to that for CREATE
VIEW
see Section 13.1.23, “CREATE VIEW Syntax”). This statement
requires the CREATE VIEW
and
DROP
privileges for the view, and
some privilege for each column referred to in the
SELECT
statement.
ALTER VIEW
is permitted only to the
definer or users with the
SET_USER_ID
or
SUPER
privilege.
CREATE {DATABASE | SCHEMA} [IF NOT EXISTS]db_name
[create_specification
] ...create_specification
: [DEFAULT] CHARACTER SET [=]charset_name
| [DEFAULT] COLLATE [=]collation_name
| DEFAULT ENCRYPTION [=] {'Y' | 'N'}
CREATE DATABASE
creates a database
with the given name. To use this statement, you need the
CREATE
privilege for the database.
CREATE
SCHEMA
is a synonym for CREATE
DATABASE
.
An error occurs if the database exists and you did not specify
IF NOT EXISTS
.
CREATE DATABASE
is not permitted
within a session that has an active LOCK
TABLES
statement.
create_specification
options specify
database characteristics. Database characteristics are stored in
the data dictionary. The CHARACTER SET
clause
specifies the default database character set. The
COLLATE
clause specifies the default database
collation. Chapter 10, Character Sets, Collations, Unicode, discusses character set and
collation names. The DEFAULT ENCRYPTION
option,
introduced in MySQL 8.0.16, defines the database encryption
default that is applied to all tables created in the database. If
the DEFAULT ENCRYPTION
option is not specified,
the default_table_encryption
setting defines the database encryption default.
A database in MySQL is implemented as a directory containing files
that correspond to tables in the database. Because there are no
tables in a database when it is initially created, the
CREATE DATABASE
statement creates
only a directory under the MySQL data directory. Rules for
permissible database names are given in
Section 9.2, “Schema Object Names”. If a database name contains special
characters, the name for the database directory contains encoded
versions of those characters as described in
Section 9.2.3, “Mapping of Identifiers to File Names”.
Creating a database directory by manually creating a directory under the data directory (for example, with mkdir) is unsupported in MySQL 8.0.
You can also use the mysqladmin program to create databases. See Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”.
CREATE [DEFINER = {user
| CURRENT_USER }] EVENT [IF NOT EXISTS]event_name
ON SCHEDULEschedule
[ON COMPLETION [NOT] PRESERVE] [ENABLE | DISABLE | DISABLE ON SLAVE] [COMMENT 'string
'] DOevent_body
;schedule
: ATtimestamp
[+ INTERVALinterval
] ... | EVERYinterval
[STARTStimestamp
[+ INTERVALinterval
] ...] [ENDStimestamp
[+ INTERVALinterval
] ...]interval
:quantity
{YEAR | QUARTER | MONTH | DAY | HOUR | MINUTE | WEEK | SECOND | YEAR_MONTH | DAY_HOUR | DAY_MINUTE | DAY_SECOND | HOUR_MINUTE | HOUR_SECOND | MINUTE_SECOND}
This statement creates and schedules a new event. The event will not run unless the Event Scheduler is enabled. For information about checking Event Scheduler status and enabling it if necessary, see Section 24.4.2, “Event Scheduler Configuration”.
CREATE EVENT
requires the
EVENT
privilege for the schema in
which the event is to be created. It might also require the
SET_USER_ID
or
SUPER
privilege, depending on the
DEFINER
value, as described later in this
section.
The minimum requirements for a valid CREATE
EVENT
statement are as follows:
The keywords CREATE EVENT
plus
an event name, which uniquely identifies the event in a
database schema.
An ON SCHEDULE
clause, which determines
when and how often the event executes.
A DO
clause, which contains the
SQL statement to be executed by an event.
This is an example of a minimal CREATE
EVENT
statement:
CREATE EVENT myevent ON SCHEDULE AT CURRENT_TIMESTAMP + INTERVAL 1 HOUR DO UPDATE myschema.mytable SET mycol = mycol + 1;
The previous statement creates an event named
myevent
. This event executes once—one
hour following its creation—by running an SQL statement that
increments the value of the myschema.mytable
table's mycol
column by 1.
The event_name
must be a valid MySQL
identifier with a maximum length of 64 characters. Event names are
not case-sensitive, so you cannot have two events named
myevent
and MyEvent
in the
same schema. In general, the rules governing event names are the
same as those for names of stored routines. See
Section 9.2, “Schema Object Names”.
An event is associated with a schema. If no schema is indicated as
part of event_name
, the default
(current) schema is assumed. To create an event in a specific
schema, qualify the event name with a schema using
syntax.
schema_name
.event_name
The DEFINER
clause specifies the MySQL account
to be used when checking access privileges at event execution
time. If a user
value is given, it
should be a MySQL account specified as
'
,
user_name
'@'host_name
'CURRENT_USER
, or
CURRENT_USER()
. The default
DEFINER
value is the user who executes the
CREATE EVENT
statement. This is the
same as specifying DEFINER = CURRENT_USER
explicitly.
If you specify the DEFINER
clause, these rules
determine the valid DEFINER
user values:
If you do not have the
SET_USER_ID
or
SUPER
privilege, the only
permitted user
value is your own
account, either specified literally or by using
CURRENT_USER
. You cannot set
the definer to some other account.
If you have the SET_USER_ID
or
SUPER
privilege, you can
specify any syntactically valid account name. If the account
does not exist, a warning is generated.
Although it is possible to create an event with a nonexistent
DEFINER
account, an error occurs at event
execution time if the account does not exist.
For more information about event security, see Section 24.6, “Access Control for Stored Programs and Views”.
Within an event, the CURRENT_USER()
function returns the account used to check privileges at event
execution time, which is the DEFINER
user. For
information about user auditing within events, see
Section 6.3.13, “SQL-Based MySQL Account Activity Auditing”.
IF NOT EXISTS
has the same meaning for
CREATE EVENT
as for
CREATE TABLE
: If an event named
event_name
already exists in the same
schema, no action is taken, and no error results. (However, a
warning is generated in such cases.)
The ON SCHEDULE
clause determines when, how
often, and for how long the event_body
defined for the event repeats. This clause takes one of two forms:
AT
is
used for a one-time event. It specifies that the event
executes one time only at the date and time given by
timestamp
timestamp
, which must include both
the date and time, or must be an expression that resolves to a
datetime value. You may use a value of either the
DATETIME
or
TIMESTAMP
type for this
purpose. If the date is in the past, a warning occurs, as
shown here:
mysql>SELECT NOW();
+---------------------+ | NOW() | +---------------------+ | 2006-02-10 23:59:01 | +---------------------+ 1 row in set (0.04 sec) mysql>CREATE EVENT e_totals
->ON SCHEDULE AT '2006-02-10 23:59:00'
->DO INSERT INTO test.totals VALUES (NOW());
Query OK, 0 rows affected, 1 warning (0.00 sec) mysql>SHOW WARNINGS\G
*************************** 1. row *************************** Level: Note Code: 1588 Message: Event execution time is in the past and ON COMPLETION NOT PRESERVE is set. The event was dropped immediately after creation.
CREATE EVENT
statements which
are themselves invalid—for whatever reason—fail
with an error.
You may use CURRENT_TIMESTAMP
to specify the current date and time. In such a case, the
event acts as soon as it is created.
To create an event which occurs at some point in the future
relative to the current date and time—such as that
expressed by the phrase “three weeks from
now”—you can use the optional clause +
INTERVAL
. The
interval
interval
portion consists of two
parts, a quantity and a unit of time, and follows the syntax
rules described in Temporal Intervals,
except that you cannot use any units keywords that involving
microseconds when defining an event. With some interval types,
complex time units may be used. For example, “two
minutes and ten seconds” can be expressed as +
INTERVAL '2:10' MINUTE_SECOND
.
You can also combine intervals. For example, AT
CURRENT_TIMESTAMP + INTERVAL 3 WEEK + INTERVAL 2 DAY
is equivalent to “three weeks and two days from
now”. Each portion of such a clause must begin with
+ INTERVAL
.
To repeat actions at a regular interval, use an
EVERY
clause. The EVERY
keyword is followed by an interval
as described in the previous discussion of the
AT
keyword. (+ INTERVAL
is not used with
EVERY
.) For example, EVERY 6
WEEK
means “every six weeks”.
Although + INTERVAL
clauses are not
permitted in an EVERY
clause, you can use
the same complex time units permitted in a +
INTERVAL
.
An EVERY
clause may contain an optional
STARTS
clause. STARTS
is
followed by a timestamp
value that
indicates when the action should begin repeating, and may also
use + INTERVAL
to specify an
amount of time “from now”. For example,
interval
EVERY 3 MONTH STARTS CURRENT_TIMESTAMP + INTERVAL 1
WEEK
means “every three months, beginning one
week from now”. Similarly, you can express “every
two weeks, beginning six hours and fifteen minutes from
now” as EVERY 2 WEEK STARTS CURRENT_TIMESTAMP
+ INTERVAL '6:15' HOUR_MINUTE
. Not specifying
STARTS
is the same as using STARTS
CURRENT_TIMESTAMP
—that is, the action
specified for the event begins repeating immediately upon
creation of the event.
An EVERY
clause may contain an optional
ENDS
clause. The ENDS
keyword is followed by a timestamp
value that tells MySQL when the event should stop repeating.
You may also use + INTERVAL
with
interval
ENDS
; for instance, EVERY 12 HOUR
STARTS CURRENT_TIMESTAMP + INTERVAL 30 MINUTE ENDS
CURRENT_TIMESTAMP + INTERVAL 4 WEEK
is equivalent to
“every twelve hours, beginning thirty minutes from now,
and ending four weeks from now”. Not using
ENDS
means that the event continues
executing indefinitely.
ENDS
supports the same syntax for complex
time units as STARTS
does.
You may use STARTS
,
ENDS
, both, or neither in an
EVERY
clause.
If a repeating event does not terminate within its scheduling
interval, the result may be multiple instances of the event
executing simultaneously. If this is undesirable, you should
institute a mechanism to prevent simultaneous instances. For
example, you could use the
GET_LOCK()
function, or row or
table locking.
The ON SCHEDULE
clause may use expressions
involving built-in MySQL functions and user variables to obtain
any of the timestamp
or
interval
values which it contains. You
may not use stored functions or user-defined functions in such
expressions, nor may you use any table references; however, you
may use SELECT FROM DUAL
. This is true for both
CREATE EVENT
and
ALTER EVENT
statements. References
to stored functions, user-defined functions, and tables in such
cases are specifically not permitted, and fail with an error (see
Bug #22830).
Times in the ON SCHEDULE
clause are interpreted
using the current session
time_zone
value. This becomes the
event time zone; that is, the time zone that is used for event
scheduling and is in effect within the event as it executes. These
times are converted to UTC and stored along with the event time
zone in the mysql.event
table. This enables
event execution to proceed as defined regardless of any subsequent
changes to the server time zone or daylight saving time effects.
For additional information about representation of event times,
see Section 24.4.4, “Event Metadata”. See also
Section 13.7.6.18, “SHOW EVENTS Syntax”, and Section 25.10, “The INFORMATION_SCHEMA EVENTS Table”.
Normally, once an event has expired, it is immediately dropped.
You can override this behavior by specifying ON
COMPLETION PRESERVE
. Using ON COMPLETION NOT
PRESERVE
merely makes the default nonpersistent behavior
explicit.
You can create an event but prevent it from being active using the
DISABLE
keyword. Alternatively, you can use
ENABLE
to make explicit the default status,
which is active. This is most useful in conjunction with
ALTER EVENT
(see
Section 13.1.3, “ALTER EVENT Syntax”).
A third value may also appear in place of
ENABLE
or DISABLE
;
DISABLE ON SLAVE
is set for the status of an
event on a replication slave to indicate that the event was
created on the master and replicated to the slave, but is not
executed on the slave. See
Section 17.4.1.16, “Replication of Invoked Features”.
You may supply a comment for an event using a
COMMENT
clause.
comment
may be any string of up to 64
characters that you wish to use for describing the event. The
comment text, being a string literal, must be surrounded by
quotation marks.
The DO
clause specifies an action
carried by the event, and consists of an SQL statement. Nearly any
valid MySQL statement that can be used in a stored routine can
also be used as the action statement for a scheduled event. (See
Section C.1, “Restrictions on Stored Programs”.) For example, the
following event e_hourly
deletes all rows from
the sessions
table once per hour, where this
table is part of the site_activity
schema:
CREATE EVENT e_hourly ON SCHEDULE EVERY 1 HOUR COMMENT 'Clears out sessions table each hour.' DO DELETE FROM site_activity.sessions;
MySQL stores the sql_mode
system
variable setting in effect when an event is created or altered,
and always executes the event with this setting in force,
regardless of the current server SQL mode when the event
begins executing.
A CREATE EVENT
statement that
contains an ALTER EVENT
statement
in its DO
clause appears to
succeed; however, when the server attempts to execute the
resulting scheduled event, the execution fails with an error.
Statements such as SELECT
or
SHOW
that merely return a result
set have no effect when used in an event; the output from these
is not sent to the MySQL Monitor, nor is it stored anywhere.
However, you can use statements such as
SELECT ...
INTO
and
INSERT INTO ...
SELECT
that store a result. (See the next example in
this section for an instance of the latter.)
The schema to which an event belongs is the default schema for
table references in the DO
clause.
Any references to tables in other schemas must be qualified with
the proper schema name.
As with stored routines, you can use compound-statement syntax in
the DO
clause by using the
BEGIN
and END
keywords, as
shown here:
delimiter | CREATE EVENT e_daily ON SCHEDULE EVERY 1 DAY COMMENT 'Saves total number of sessions then clears the table each day' DO BEGIN INSERT INTO site_activity.totals (time, total) SELECT CURRENT_TIMESTAMP, COUNT(*) FROM site_activity.sessions; DELETE FROM site_activity.sessions; END | delimiter ;
This example uses the delimiter
command to
change the statement delimiter. See
Section 24.1, “Defining Stored Programs”.
More complex compound statements, such as those used in stored routines, are possible in an event. This example uses local variables, an error handler, and a flow control construct:
delimiter | CREATE EVENT e ON SCHEDULE EVERY 5 SECOND DO BEGIN DECLARE v INTEGER; DECLARE CONTINUE HANDLER FOR SQLEXCEPTION BEGIN END; SET v = 0; WHILE v < 5 DO INSERT INTO t1 VALUES (0); UPDATE t2 SET s1 = s1 + 1; SET v = v + 1; END WHILE; END | delimiter ;
There is no way to pass parameters directly to or from events; however, it is possible to invoke a stored routine with parameters within an event:
CREATE EVENT e_call_myproc ON SCHEDULE AT CURRENT_TIMESTAMP + INTERVAL 1 DAY DO CALL myproc(5, 27);
If an event's definer has privileges sufficient to set global system variables (see Section 5.1.9.1, “System Variable Privileges”), the event can read and write global variables. As granting such privileges entails a potential for abuse, extreme care must be taken in doing so.
Generally, any statements that are valid in stored routines may be used for action statements executed by events. For more information about statements permissible within stored routines, see Section 24.2.1, “Stored Routine Syntax”. You can create an event as part of a stored routine, but an event cannot be created by another event.
The CREATE FUNCTION
statement is
used to create stored functions and user-defined functions (UDFs):
For information about creating stored functions, see Section 13.1.17, “CREATE PROCEDURE and CREATE FUNCTION Syntax”.
For information about creating user-defined functions, see Section 13.7.4.1, “CREATE FUNCTION Syntax for User-Defined Functions”.
CREATE [UNIQUE | FULLTEXT | SPATIAL] INDEXindex_name
[index_type
] ONtbl_name
(key_part
,...) [index_option
] [algorithm_option
|lock_option
] ...key_part
: {col_name
[(length
)] | (expr
)} [ASC | DESC]index_option
: KEY_BLOCK_SIZE [=]value
|index_type
| WITH PARSERparser_name
| COMMENT 'string
' | {VISIBLE | INVISIBLE}index_type
: USING {BTREE | HASH}algorithm_option
: ALGORITHM [=] {DEFAULT | INPLACE | COPY}lock_option
: LOCK [=] {DEFAULT | NONE | SHARED | EXCLUSIVE}
Normally, you create all indexes on a table at the time the table
itself is created with CREATE
TABLE
. See Section 13.1.20, “CREATE TABLE Syntax”. This
guideline is especially important for
InnoDB
tables, where the primary key
determines the physical layout of rows in the data file.
CREATE INDEX
enables you to add
indexes to existing tables.
CREATE INDEX
is mapped to an
ALTER TABLE
statement to create
indexes. See Section 13.1.9, “ALTER TABLE Syntax”.
CREATE INDEX
cannot be used to
create a PRIMARY KEY
; use
ALTER TABLE
instead. For more
information about indexes, see Section 8.3.1, “How MySQL Uses Indexes”.
InnoDB
supports secondary indexes on
virtual columns. For more information, see
Section 13.1.20.10, “Secondary Indexes and Generated Columns”.
When the innodb_stats_persistent
setting is enabled, run the ANALYZE
TABLE
statement for an
InnoDB
table after creating an index
on that table.
An index specification of the form
(
creates an
index with multiple key parts. Index key values are formed by
concatenating the values of the given key parts. For example
key_part1
,
key_part2
, ...)(col1, col2, col3)
specifies a multiple-column
index with index keys consisting of values from
col1
, col2
, and
col3
.
A key_part
specification can end with
ASC
or DESC
to specify
whether index values are stored in ascending or descending order.
The default is ascending if no order specifier is given.
ASC
and DESC
are not
permitted for HASH
indexes. As of MySQL 8.0.12,
ASC
and DESC
are not
permitted for SPATIAL
indexes.
The following sections describe different aspects of the
CREATE INDEX
statement:
For string columns, indexes can be created that use only the
leading part of column values, using
syntax to specify an index prefix length:
col_name
(length
)
Prefixes can be specified for
CHAR
,
VARCHAR
,
BINARY
, and
VARBINARY
key parts.
Prefixes must be specified for
BLOB
and
TEXT
key parts. Additionally,
BLOB
and
TEXT
columns can be indexed
only for InnoDB
,
MyISAM
, and BLACKHOLE
tables.
Prefix limits are measured in bytes.
However, prefix lengths for index
specifications in CREATE
TABLE
, ALTER TABLE
,
and CREATE INDEX
statements
are interpreted as number of characters for nonbinary string
types (CHAR
,
VARCHAR
,
TEXT
) and number of bytes for
binary string types (BINARY
,
VARBINARY
,
BLOB
). Take this into account
when specifying a prefix length for a nonbinary string
column that uses a multibyte character set.
Prefix support and lengths of prefixes (where supported) are
storage engine dependent. For example, a prefix can be up to
767 bytes long for InnoDB
tables that use the
REDUNDANT
or
COMPACT
row format. The prefix length limit is 3072 bytes for
InnoDB
tables that use the
DYNAMIC
or
COMPRESSED
row format. For MyISAM
tables,
the prefix length limit is 1000 bytes. The
NDB
storage engine does not
support prefixes (see
Section 22.1.7.6, “Unsupported or Missing Features in NDB Cluster”).
If a specified index prefix exceeds the maximum column data type
size, CREATE INDEX
handles the
index as follows:
For a nonunique index, either an error occurs (if strict SQL mode is enabled), or the index length is reduced to lie within the maximum column data type size and a warning is produced (if strict SQL mode is not enabled).
For a unique index, an error occurs regardless of SQL mode because reducing the index length might enable insertion of nonunique entries that do not meet the specified uniqueness requirement.
The statement shown here creates an index using the first 10
characters of the name
column (assuming that
name
has a nonbinary string type):
CREATE INDEX part_of_name ON customer (name(10));
If names in the column usually differ in the first 10
characters, lookups performed using this index should not be
much slower than using an index created from the entire
name
column. Also, using column prefixes for
indexes can make the index file much smaller, which could save a
lot of disk space and might also speed up
INSERT
operations.
A “normal” index indexes column values or prefixes
of column values. For example, in the following table, the index
entry for a given t1
row includes the full
col1
value and a prefix of the
col2
value consisting of its first 10
characters:
CREATE TABLE t1 ( col1 VARCHAR(10), col2 VARCHAR(20), INDEX (col1, col2(10)) );
MySQL 8.0.13 and higher supports functional key parts that index expression values rather than column or column prefix values. Use of functional key parts enables indexing of values not stored directly in the table. Examples:
CREATE TABLE t1 (col1 INT, col2 INT, INDEX func_index ((ABS(col1)))); CREATE INDEX idx1 ON t1 ((col1 + col2)); CREATE INDEX idx2 ON t1 ((col1 + col2), (col1 - col2), col1); ALTER TABLE t1 ADD INDEX ((col1 * 40) DESC);
An index with multiple key parts can mix nonfunctional and functional key parts.
ASC
and DESC
are supported
for functional key parts.
Functional key parts must adhere to the following rules. An error occurs if a key part definition contains disallowed constructs.
In index definitions, enclose expressions within parentheses to distinguish them from columns or column prefixes. For example, this is permitted; the expressions are enclosed within parentheses:
INDEX ((col1 + col2), (col3 - col4))
This produces an error; the expressions are not enclosed within parentheses:
INDEX (col1 + col2, col3 - col4)
A functional key part cannot consist solely of a column name. For example, this is not permitted:
INDEX ((col1), (col2))
Instead, write the key parts as nonfunctional key parts, without parentheses:
INDEX (col1, col2)
A functional key part expression cannot refer to column
prefixes. For a workaround, see the discussion of
SUBSTRING()
and
CAST()
later in this section.
Functional key parts are not permitted in foreign key specifications.
For CREATE
TABLE ... LIKE
, the destination table preserves
functional key parts from the original table.
Functional indexes are implemented as hidden virtual generated columns, which has these implications:
Each functional key part counts against the limit on total number of table columns; see Section C.10.4, “Limits on Table Column Count and Row Size”.
Functional key parts inherit all restrictions that apply to generated columns. Examples:
Only functions permitted for generated columns are permitted for functional key parts.
Subqueries, parameters, variables, stored functions, and user-defined functions are not permitted.
For more information about applicable restrictions, see Section 13.1.20.9, “CREATE TABLE and Generated Columns”, and Section 13.1.9.2, “ALTER TABLE and Generated Columns”.
The virtual generated column itself requires no storage. The index itself takes up storage space as any other index.
UNIQUE
is supported for indexes that include
functional key parts. However, primary keys cannot include
functional key parts. A primary key requires the generated
column to be stored, but functional key parts are implemented as
virtual generated columns, not stored generated columns.
SPATIAL
and FULLTEXT
indexes cannot have functional key parts.
If a table contains no primary key, InnoDB
automatically promotes the first UNIQUE NOT
NULL
index to the primary key. This is not supported
for UNIQUE NOT NULL
indexes that have
functional key parts.
Nonfunctional indexes raise a warning if there are duplicate indexes. Indexes that contain functional key parts do not have this feature.
To remove a column that is referenced by a functional key part, the index must be removed first. Otherwise, an error occurs.
Although nonfunctional key parts support a prefix length
specification, this is not possible for functional key parts.
The solution is to use
SUBSTRING()
(or
CAST()
, as described later in
this section). For a functional key part containing the
SUBSTRING()
function to be used
in a query, the WHERE
clause must contain
SUBSTRING()
with the same
arguments. In the following example, only the second
SELECT
is able to use the index
because that is the only query in which the arguments to
SUBSTRING()
match the index
specification:
CREATE TABLE tbl ( col1 LONGTEXT, INDEX idx1 ((SUBSTRING(col1, 1, 10))) ); SELECT * FROM tbl WHERE SUBSTRING(col1, 1, 9) = '123456789'; SELECT * FROM tbl WHERE SUBSTRING(col1, 1, 10) = '1234567890';
Functional key parts enable indexing of values that cannot be
indexed otherwise, such as JSON
values. However, this must be done correctly to achieve the
desired effect. For example, this syntax does not work:
CREATE TABLE employees ( data JSON, INDEX ((data->>'$.name')) );
The syntax fails because:
The
->>
operator translates into
JSON_UNQUOTE(JSON_EXTRACT(...))
.
JSON_UNQUOTE()
returns a
value with a data type of
LONGTEXT
, and the hidden
generated column thus is assigned the same data type.
MySQL cannot index LONGTEXT
columns specified without a prefix length on the key part,
and prefix lengths are not permitted in functional key
parts.
To index the JSON
column, you could try using
the CAST()
function as follows:
CREATE TABLE employees ( data JSON, INDEX ((CAST(data->>'$.name' AS CHAR(30)))) );
The hidden generated column is assigned the
VARCHAR(30)
data type, which can
be indexed. But this approach produces a new issue when trying
to use the index:
CAST()
returns a string with
the collation utf8mb4_0900_ai_ci
(the
server default collation).
JSON_UNQUOTE()
returns a
string with the collation utf8mb4_bin
(hard coded).
As a result, there is a collation mismatch between the indexed
expression in the preceding table definition and the
WHERE
clause expression in the following
query:
SELECT * FROM employees WHERE data->>'$.name' = 'James';
The index is not used because the expressions in the query and
the index differ. To support this kind of scenario for
functional key parts, the optimizer automatically strips
CAST()
when looking for an index
to use, but only if the collation of the
indexed expression matches that of the query expression. For an
index with a functional key part to be used, either of the
following two solutions work (although they differ somewhat in
effect):
Solution 1. Assign the indexed expression the same collation
as JSON_UNQUOTE()
:
CREATE TABLE employees ( data JSON, INDEX idx ((CAST(data->>"$.name" AS CHAR(30)) COLLATE utf8mb4_bin)) ); INSERT INTO employees VALUES ('{ "name": "james", "salary": 9000 }'), ('{ "name": "James", "salary": 10000 }'), ('{ "name": "Mary", "salary": 12000 }'), ('{ "name": "Peter", "salary": 8000 }'); SELECT * FROM employees WHERE data->>'$.name' = 'James';
The ->>
operator is the same as
JSON_UNQUOTE(JSON_EXTRACT(...))
, and
JSON_UNQUOTE()
returns a string with
collation utf8mb4_bin
. The comparison is
thus case sensitive, and only one row matches:
+------------------------------------+ | data | +------------------------------------+ | {"name": "James", "salary": 10000} | +------------------------------------+
Solution 2. Specify the full expression in the query:
CREATE TABLE employees ( data JSON, INDEX idx ((CAST(data->>"$.name" AS CHAR(30)))) ); INSERT INTO employees VALUES ('{ "name": "james", "salary": 9000 }'), ('{ "name": "James", "salary": 10000 }'), ('{ "name": "Mary", "salary": 12000 }'), ('{ "name": "Peter", "salary": 8000 }'); SELECT * FROM employees WHERE CAST(data->>'$.name' AS CHAR(30)) = 'James';
CAST()
returns a string with collation
utf8mb4_0900_ai_ci
, so the comparison
case insensitive and two rows match:
+------------------------------------+ | data | +------------------------------------+ | {"name": "james", "salary": 9000} | | {"name": "James", "salary": 10000} | +------------------------------------+
Be aware that although the the optimizer supports automatically
stripping CAST()
with indexed
generated columns, the following approach does not work because
it produces a different result with and without an index
(Bug#27337092):
mysql>CREATE TABLE employees (
data JSON,
generated_col VARCHAR(30) AS (CAST(data->>'$.name' AS CHAR(30)))
);
Query OK, 0 rows affected, 1 warning (0.03 sec) mysql>INSERT INTO employees (data)
VALUES ('{"name": "james"}'), ('{"name": "James"}');
Query OK, 2 rows affected, 1 warning (0.01 sec) Records: 2 Duplicates: 0 Warnings: 1 mysql>SELECT * FROM employees WHERE data->>'$.name' = 'James';
+-------------------+---------------+ | data | generated_col | +-------------------+---------------+ | {"name": "James"} | James | +-------------------+---------------+ 1 row in set (0.00 sec) mysql>ALTER TABLE employees ADD INDEX idx (generated_col);
Query OK, 0 rows affected, 1 warning (0.03 sec) Records: 0 Duplicates: 0 Warnings: 1 mysql>SELECT * FROM employees WHERE data->>'$.name' = 'James';
+-------------------+---------------+ | data | generated_col | +-------------------+---------------+ | {"name": "james"} | james | | {"name": "James"} | James | +-------------------+---------------+ 2 rows in set (0.01 sec)
A UNIQUE
index creates a constraint such that
all values in the index must be distinct. An error occurs if you
try to add a new row with a key value that matches an existing
row. If you specify a prefix value for a column in a
UNIQUE
index, the column values must be
unique within the prefix length. A UNIQUE
index permits multiple NULL
values for
columns that can contain NULL
.
If a table has a PRIMARY KEY
or
UNIQUE NOT NULL
index that consists of a
single column that has an integer type, you can use
_rowid
to refer to the indexed column in
SELECT
statements, as follows:
_rowid
refers to the PRIMARY
KEY
column if there is a PRIMARY
KEY
consisting of a single integer column. If
there is a PRIMARY KEY
but it does not
consist of a single integer column,
_rowid
cannot be used.
Otherwise, _rowid
refers to the column in
the first UNIQUE NOT NULL
index if that
index consists of a single integer column. If the first
UNIQUE NOT NULL
index does not consist of
a single integer column, _rowid
cannot be
used.
FULLTEXT
indexes are supported only for
InnoDB
and
MyISAM
tables and can include only
CHAR
,
VARCHAR
, and
TEXT
columns. Indexing always
happens over the entire column; column prefix indexing is not
supported and any prefix length is ignored if specified. See
Section 12.9, “Full-Text Search Functions”, for details of operation.
The MyISAM
,
InnoDB
,
NDB
, and
ARCHIVE
storage engines support
spatial columns such as POINT
and
GEOMETRY
.
(Section 11.5, “Spatial Data Types”, describes the spatial data
types.) However, support for spatial column indexing varies
among engines. Spatial and nonspatial indexes on spatial columns
are available according to the following rules.
Spatial indexes on spatial columns have these characteristics:
Available only for InnoDB
and
MyISAM
tables. Specifying
SPATIAL INDEX
for other storage engines
results in an error.
As of MySQL 8.0.12, an index on a spatial column
must be a SPATIAL
index. The SPATIAL
keyword is thus
optional but implicit for creating an index on a spatial
column.
Available for single spatial columns only. A spatial index cannot be created over multiple spatial columns.
Indexed columns must be NOT NULL
.
Column prefix lengths are prohibited. The full width of each column is indexed.
Not permitted for a primary key or unique index.
Nonspatial indexes on spatial columns (created with
INDEX
, UNIQUE
, or
PRIMARY KEY
) have these characteristics:
Permitted for any storage engine that supports spatial
columns except ARCHIVE
.
Columns can be NULL
unless the index is a
primary key.
The index type for a non-SPATIAL
index
depends on the storage engine. Currently, B-tree is used.
Permitted for a column that can have NULL
values only for InnoDB
,
MyISAM
, and
MEMORY
tables.
Following the key part list, index options can be given. An
index_option
value can be any of the
following:
KEY_BLOCK_SIZE [=]
value
For MyISAM
tables,
KEY_BLOCK_SIZE
optionally specifies the
size in bytes to use for index key blocks. The value is
treated as a hint; a different size could be used if
necessary. A KEY_BLOCK_SIZE
value
specified for an individual index definition overrides a
table-level KEY_BLOCK_SIZE
value.
KEY_BLOCK_SIZE
is not supported at the
index level for InnoDB
tables.
See Section 13.1.20, “CREATE TABLE Syntax”.
index_type
Some storage engines permit you to specify an index type when creating an index. For example:
CREATE TABLE lookup (id INT) ENGINE = MEMORY; CREATE INDEX id_index ON lookup (id) USING BTREE;
Table 13.1, “Index Types Per Storage Engine”
shows the permissible index type values supported by
different storage engines. Where multiple index types are
listed, the first one is the default when no index type
specifier is given. Storage engines not listed in the table
do not support an index_type
clause in index definitions.
The index_type
clause cannot be
used for FULLTEXT INDEX
or (prior to
MySQL 8.0.12) SPATIAL INDEX
specifications. Full-text index implementation is storage
engine dependent. Spatial indexes are implemented as R-tree
indexes.
If you specify an index type that is not valid for a given
storage engine, but another index type is available that the
engine can use without affecting query results, the engine
uses the available type. The parser recognizes
RTREE
as a type name. As of MySQL 8.0.12,
this is permitted only for SPATIAL
indexes. Prior to 8.0.12, RTREE
cannot be
specified for any storage engine.
BTREE
indexes are implemented by the
NDB
storage engine as T-tree
indexes.
For indexes on NDB
table
columns, the USING
option can be
specified only for a unique index or primary key.
USING HASH
prevents the creation of an
ordered index; otherwise, creating a unique index or
primary key on an NDB
table
automatically results in the creation of both an ordered
index and a hash index, each of which indexes the same set
of columns.
For unique indexes that include one or more
NULL
columns of an
NDB
table, the hash index can
be used only to look up literal values, which means that
IS [NOT] NULL
conditions require a full
scan of the table. One workaround is to make sure that a
unique index using one or more NULL
columns on such a table is always created in such a way
that it includes the ordered index; that is, avoid
employing USING HASH
when creating the
index.
If you specify an index type that is not valid for a given
storage engine, but another index type is available that the
engine can use without affecting query results, the engine
uses the available type. The parser recognizes
RTREE
as a type name, but currently this
cannot be specified for any storage engine.
Use of the index_type
option
before the ON
clause is
deprecated; support for use of the option in this position
will be removed in a future MySQL release. If an
tbl_name
index_type
option is given in
both the earlier and later positions, the final option
applies.
TYPE
is recognized as a synonym for type_name
USING
. However,
type_name
USING
is the preferred form.
The following tables show index characteristics for the
storage engines that support the
index_type
option.
Table 13.2 InnoDB Storage Engine Index Characteristics
Index Class | Index Type | Stores NULL VALUES | Permits Multiple NULL Values | IS NULL Scan Type | IS NOT NULL Scan Type |
---|---|---|---|---|---|
Primary key | BTREE |
No | No | N/A | N/A |
Unique | BTREE |
Yes | Yes | Index | Index |
Key | BTREE |
Yes | Yes | Index | Index |
FULLTEXT |
N/A | Yes | Yes | Table | Table |
SPATIAL |
N/A | No | No | N/A | N/A |
Table 13.3 MyISAM Storage Engine Index Characteristics
Index Class | Index Type | Stores NULL VALUES | Permits Multiple NULL Values | IS NULL Scan Type | IS NOT NULL Scan Type |
---|---|---|---|---|---|
Primary key | BTREE |
No | No | N/A | N/A |
Unique | BTREE |
Yes | Yes | Index | Index |
Key | BTREE |
Yes | Yes | Index | Index |
FULLTEXT |
N/A | Yes | Yes | Table | Table |
SPATIAL |
N/A | No | No | N/A | N/A |
Table 13.4 MEMORY Storage Engine Index Characteristics
Index Class | Index Type | Stores NULL VALUES | Permits Multiple NULL Values | IS NULL Scan Type | IS NOT NULL Scan Type |
---|---|---|---|---|---|
Primary key | BTREE |
No | No | N/A | N/A |
Unique | BTREE |
Yes | Yes | Index | Index |
Key | BTREE |
Yes | Yes | Index | Index |
Primary key | HASH |
No | No | N/A | N/A |
Unique | HASH |
Yes | Yes | Index | Index |
Key | HASH |
Yes | Yes | Index | Index |
Table 13.5 NDB Storage Engine Index Characteristics
Index Class | Index Type | Stores NULL VALUES | Permits Multiple NULL Values | IS NULL Scan Type | IS NOT NULL Scan Type |
---|---|---|---|---|---|
Primary key | BTREE |
No | No | Index | Index |
Unique | BTREE |
Yes | Yes | Index | Index |
Key | BTREE |
Yes | Yes | Index | Index |
Primary key | HASH |
No | No | Table (see note 1) | Table (see note 1) |
Unique | HASH |
Yes | Yes | Table (see note 1) | Table (see note 1) |
Key | HASH |
Yes | Yes | Table (see note 1) | Table (see note 1) |
Table note:
1. USING HASH
prevents creation of an
implicit ordered index.
WITH PARSER
parser_name
This option can be used only with
FULLTEXT
indexes. It associates a parser
plugin with the index if full-text indexing and searching
operations need special handling.
InnoDB
and
MyISAM
support full-text parser
plugins. See Full-Text Parser Plugins and
Section 29.2.4.4, “Writing Full-Text Parser Plugins” for more
information.
COMMENT
'
string
'
Index definitions can include an optional comment of up to 1024 characters.
The
MERGE_THRESHOLD
for index pages can be configured for individual indexes
using the index_option
COMMENT
clause of the
CREATE INDEX
statement. For
example:
CREATE TABLE t1 (id INT); CREATE INDEX id_index ON t1 (id) COMMENT 'MERGE_THRESHOLD=40';
If the page-full percentage for an index page falls below
the MERGE_THRESHOLD
value when a row is
deleted or when a row is shortened by an update operation,
InnoDB
attempts to merge the
index page with a neighboring index page. The default
MERGE_THRESHOLD
value is 50, which is the
previously hardcoded value.
MERGE_THRESHOLD
can also be defined at
the index level and table level using
CREATE TABLE
and
ALTER TABLE
statements. For
more information, see
Section 15.8.11, “Configuring the Merge Threshold for Index Pages”.
VISIBLE
, INVISIBLE
Specify index visibility. Indexes are visible by default. An invisible index is not used by the optimizer. Specification of index visibility applies to indexes other than primary keys (either explicit or implicit). For more information, see Section 8.3.12, “Invisible Indexes”.
ALGORITHM
and LOCK
clauses
may be given to influence the table copying method and level of
concurrency for reading and writing the table while its indexes
are being modified. They have the same meaning as for the
ALTER TABLE
statement. For more
information, see Section 13.1.9, “ALTER TABLE Syntax”
NDB Cluster supports online operations using the same
ALGORITHM=INPLACE
syntax used with the
standard MySQL Server. See
Section 22.5.14, “Online Operations with ALTER TABLE in NDB Cluster”, for more
information.
CREATE LOGFILE GROUPlogfile_group
ADD UNDOFILE 'undo_file
' [INITIAL_SIZE [=]initial_size
] [UNDO_BUFFER_SIZE [=]undo_buffer_size
] [REDO_BUFFER_SIZE [=]redo_buffer_size
] [NODEGROUP [=]nodegroup_id
] [WAIT] [COMMENT [=] 'string
'] ENGINE [=]engine_name
This statement creates a new log file group named
logfile_group
having a single
UNDO
file named
'undo_file
'. A
CREATE LOGFILE GROUP
statement has
one and only one ADD UNDOFILE
clause. For rules
covering the naming of log file groups, see
Section 9.2, “Schema Object Names”.
All NDB Cluster Disk Data objects share the same namespace. This means that each Disk Data object must be uniquely named (and not merely each Disk Data object of a given type). For example, you cannot have a tablespace and a log file group with the same name, or a tablespace and a data file with the same name.
There can be only one log file group per NDB Cluster instance at any given time.
The optional INITIAL_SIZE
parameter sets the
UNDO
file's initial size; if not specified, it
defaults to 128M
(128 megabytes). The optional
UNDO_BUFFER_SIZE
parameter sets the size used
by the UNDO
buffer for the log file group; The
default value for UNDO_BUFFER_SIZE
is
8M
(eight megabytes); this value cannot exceed
the amount of system memory available. Both of these parameters
are specified in bytes. You may optionally follow either or both
of these with a one-letter abbreviation for an order of magnitude,
similar to those used in my.cnf
. Generally,
this is one of the letters M
(for megabytes)
or G
(for gigabytes).
Memory used for UNDO_BUFFER_SIZE
comes from the
global pool whose size is determined by the value of the
SharedGlobalMemory
data
node configuration parameter. This includes any default value
implied for this option by the setting of the
InitialLogFileGroup
data
node configuration parameter.
The maximum permitted for UNDO_BUFFER_SIZE
is
629145600 (600 MB).
On 32-bit systems, the maximum supported value for
INITIAL_SIZE
is 4294967296 (4 GB). (Bug #29186)
The minimum allowed value for INITIAL_SIZE
is
1048576 (1 MB).
The ENGINE
option determines the storage engine
to be used by this log file group, with
engine_name
being the name of the
storage engine. In MySQL 8.0, this must be
NDB
(or
NDBCLUSTER
). If
ENGINE
is not set, MySQL tries to use the
engine specified by the
default_storage_engine
server
system variable (formerly
storage_engine
). In any case, if
the engine is not specified as NDB
or
NDBCLUSTER
, the CREATE
LOGFILE GROUP
statement appears to succeed but actually
fails to create the log file group, as shown here:
mysql>CREATE LOGFILE GROUP lg1
->ADD UNDOFILE 'undo.dat' INITIAL_SIZE = 10M;
Query OK, 0 rows affected, 1 warning (0.00 sec) mysql>SHOW WARNINGS;
+-------+------+------------------------------------------------------------------------------------------------+ | Level | Code | Message | +-------+------+------------------------------------------------------------------------------------------------+ | Error | 1478 | Table storage engine 'InnoDB' does not support the create option 'TABLESPACE or LOGFILE GROUP' | +-------+------+------------------------------------------------------------------------------------------------+ 1 row in set (0.00 sec) mysql>DROP LOGFILE GROUP lg1 ENGINE = NDB;
ERROR 1529 (HY000): Failed to drop LOGFILE GROUP mysql>CREATE LOGFILE GROUP lg1
->ADD UNDOFILE 'undo.dat' INITIAL_SIZE = 10M
->ENGINE = NDB;
Query OK, 0 rows affected (2.97 sec)
The fact that the CREATE LOGFILE GROUP
statement does not actually return an error when a
non-NDB
storage engine is named, but rather
appears to succeed, is a known issue which we hope to address in a
future release of NDB Cluster.
REDO_BUFFER_SIZE
,
NODEGROUP
, WAIT
, and
COMMENT
are parsed but ignored, and so have no
effect in MySQL 8.0. These options are intended for
future expansion.
When used with ENGINE [=] NDB
, a log file group
and associated UNDO
log file are created on
each Cluster data node. You can verify that the
UNDO
files were created and obtain information
about them by querying the
INFORMATION_SCHEMA.FILES
table. For
example:
mysql>SELECT LOGFILE_GROUP_NAME, LOGFILE_GROUP_NUMBER, EXTRA
->FROM INFORMATION_SCHEMA.FILES
->WHERE FILE_NAME = 'undo_10.dat';
+--------------------+----------------------+----------------+ | LOGFILE_GROUP_NAME | LOGFILE_GROUP_NUMBER | EXTRA | +--------------------+----------------------+----------------+ | lg_3 | 11 | CLUSTER_NODE=3 | | lg_3 | 11 | CLUSTER_NODE=4 | +--------------------+----------------------+----------------+ 2 rows in set (0.06 sec)
CREATE LOGFILE GROUP
is useful only
with Disk Data storage for NDB Cluster. See
Section 22.5.13, “NDB Cluster Disk Data Tables”.
CREATE [DEFINER = {user
| CURRENT_USER }] PROCEDUREsp_name
([proc_parameter
[,...]]) [characteristic
...]routine_body
CREATE [DEFINER = {user
| CURRENT_USER }] FUNCTIONsp_name
([func_parameter
[,...]]) RETURNStype
[characteristic
...]routine_body
proc_parameter
: [ IN | OUT | INOUT ]param_name
type
func_parameter
:param_name
type
type
:Any valid MySQL data type
characteristic
: COMMENT 'string
' | LANGUAGE SQL | [NOT] DETERMINISTIC | { CONTAINS SQL | NO SQL | READS SQL DATA | MODIFIES SQL DATA } | SQL SECURITY { DEFINER | INVOKER }routine_body
:Valid SQL routine statement
These statements create stored routines. By default, a routine is
associated with the default database. To associate the routine
explicitly with a given database, specify the name as
db_name.sp_name
when you create it.
The CREATE FUNCTION
statement is
also used in MySQL to support UDFs (user-defined functions). See
Section 29.4, “Adding New Functions to MySQL”. A UDF can be regarded as an
external stored function. Stored functions share their namespace
with UDFs. See Section 9.2.4, “Function Name Parsing and Resolution”, for the
rules describing how the server interprets references to different
kinds of functions.
To invoke a stored procedure, use the
CALL
statement (see
Section 13.2.1, “CALL Syntax”). To invoke a stored function, refer to it
in an expression. The function returns a value during expression
evaluation.
CREATE PROCEDURE
and
CREATE FUNCTION
require the
CREATE ROUTINE
privilege. They
might also require the SET_USER_ID
or SUPER
privilege, depending on
the DEFINER
value, as described later in this
section. If binary logging is enabled, CREATE
FUNCTION
might require the
SUPER
privilege, as described in
Section 24.7, “Binary Logging of Stored Programs”.
By default, MySQL automatically grants the
ALTER ROUTINE
and
EXECUTE
privileges to the routine
creator. This behavior can be changed by disabling the
automatic_sp_privileges
system
variable. See Section 24.2.2, “Stored Routines and MySQL Privileges”.
The DEFINER
and SQL SECURITY
clauses specify the security context to be used when checking
access privileges at routine execution time, as described later in
this section.
If the routine name is the same as the name of a built-in SQL function, a syntax error occurs unless you use a space between the name and the following parenthesis when defining the routine or invoking it later. For this reason, avoid using the names of existing SQL functions for your own stored routines.
The IGNORE_SPACE
SQL mode
applies to built-in functions, not to stored routines. It is
always permissible to have spaces after a stored routine name,
regardless of whether
IGNORE_SPACE
is enabled.
The parameter list enclosed within parentheses must always be
present. If there are no parameters, an empty parameter list of
()
should be used. Parameter names are not case
sensitive.
Each parameter is an IN
parameter by default.
To specify otherwise for a parameter, use the keyword
OUT
or INOUT
before the
parameter name.
Specifying a parameter as IN
,
OUT
, or INOUT
is valid
only for a PROCEDURE
. For a
FUNCTION
, parameters are always regarded as
IN
parameters.
An IN
parameter passes a value into a
procedure. The procedure might modify the value, but the
modification is not visible to the caller when the procedure
returns. An OUT
parameter passes a value from
the procedure back to the caller. Its initial value is
NULL
within the procedure, and its value is
visible to the caller when the procedure returns. An
INOUT
parameter is initialized by the caller,
can be modified by the procedure, and any change made by the
procedure is visible to the caller when the procedure returns.
For each OUT
or INOUT
parameter, pass a user-defined variable in the
CALL
statement that invokes the
procedure so that you can obtain its value when the procedure
returns. If you are calling the procedure from within another
stored procedure or function, you can also pass a routine
parameter or local routine variable as an OUT
or INOUT
parameter. If you are calling the
procedure from within a trigger, you can also pass
NEW.
as an
col_name
OUT
or INOUT
parameter.
For information about the effect of unhandled conditions on procedure parameters, see Section 13.6.7.8, “Condition Handling and OUT or INOUT Parameters”.
Routine parameters cannot be referenced in statements prepared within the routine; see Section C.1, “Restrictions on Stored Programs”.
The following example shows a simple stored procedure that uses an
OUT
parameter:
mysql>delimiter //
mysql>CREATE PROCEDURE simpleproc (OUT param1 INT)
->BEGIN
->SELECT COUNT(*) INTO param1 FROM t;
->END//
Query OK, 0 rows affected (0.00 sec) mysql>delimiter ;
mysql>CALL simpleproc(@a);
Query OK, 0 rows affected (0.00 sec) mysql>SELECT @a;
+------+ | @a | +------+ | 3 | +------+ 1 row in set (0.00 sec)
The example uses the mysql client
delimiter
command to change the statement
delimiter from ;
to //
while
the procedure is being defined. This enables the
;
delimiter used in the procedure body to be
passed through to the server rather than being interpreted by
mysql itself. See
Section 24.1, “Defining Stored Programs”.
The RETURNS
clause may be specified only for a
FUNCTION
, for which it is mandatory. It
indicates the return type of the function, and the function body
must contain a RETURN
statement. If the
value
RETURN
statement returns a value of
a different type, the value is coerced to the proper type. For
example, if a function specifies an
ENUM
or
SET
value in the
RETURNS
clause, but the
RETURN
statement returns an
integer, the value returned from the function is the string for
the corresponding ENUM
member of
set of SET
members.
The following example function takes a parameter, performs an
operation using an SQL function, and returns the result. In this
case, it is unnecessary to use delimiter
because the function definition contains no internal
;
statement delimiters:
mysql>CREATE FUNCTION hello (s CHAR(20))
mysql>RETURNS CHAR(50) DETERMINISTIC
->RETURN CONCAT('Hello, ',s,'!');
Query OK, 0 rows affected (0.00 sec) mysql>SELECT hello('world');
+----------------+ | hello('world') | +----------------+ | Hello, world! | +----------------+ 1 row in set (0.00 sec)
Parameter types and function return types can be declared to use
any valid data type. The COLLATE
attribute can
be used if preceded by a CHARACTER SET
specification.
The routine_body
consists of a valid
SQL routine statement. This can be a simple statement such as
SELECT
or
INSERT
, or a compound statement
written using BEGIN
and END
.
Compound statements can contain declarations, loops, and other
control structure statements. The syntax for these statements is
described in Section 13.6, “Compound-Statement Syntax”.
MySQL permits routines to contain DDL statements, such as
CREATE
and DROP
. MySQL also
permits stored procedures (but not stored functions) to contain
SQL transaction statements such as
COMMIT
. Stored functions may not
contain statements that perform explicit or implicit commit or
rollback. Support for these statements is not required by the SQL
standard, which states that each DBMS vendor may decide whether to
permit them.
Statements that return a result set can be used within a stored
procedure but not within a stored function. This prohibition
includes SELECT
statements that do
not have an INTO
clause and other
statements such as var_list
SHOW
,
EXPLAIN
, and
CHECK TABLE
. For statements that
can be determined at function definition time to return a result
set, a Not allowed to return a result set from a
function
error occurs
(ER_SP_NO_RETSET
). For statements
that can be determined only at runtime to return a result set, a
PROCEDURE %s can't return a result set in the given
context
error occurs
(ER_SP_BADSELECT
).
USE
statements within stored
routines are not permitted. When a routine is invoked, an implicit
USE
is
performed (and undone when the routine terminates). The causes the
routine to have the given default database while it executes.
References to objects in databases other than the routine default
database should be qualified with the appropriate database name.
db_name
For additional information about statements that are not permitted in stored routines, see Section C.1, “Restrictions on Stored Programs”.
For information about invoking stored procedures from within programs written in a language that has a MySQL interface, see Section 13.2.1, “CALL Syntax”.
MySQL stores the sql_mode
system
variable setting in effect when a routine is created or altered,
and always executes the routine with this setting in force,
regardless of the current server SQL mode when the
routine begins executing.
The switch from the SQL mode of the invoker to that of the routine occurs after evaluation of arguments and assignment of the resulting values to routine parameters. If you define a routine in strict SQL mode but invoke it in nonstrict mode, assignment of arguments to routine parameters does not take place in strict mode. If you require that expressions passed to a routine be assigned in strict SQL mode, you should invoke the routine with strict mode in effect.
The COMMENT
characteristic is a MySQL
extension, and may be used to describe the stored routine. This
information is displayed by the SHOW CREATE
PROCEDURE
and SHOW CREATE
FUNCTION
statements.
The LANGUAGE
characteristic indicates the
language in which the routine is written. The server ignores this
characteristic; only SQL routines are supported.
A routine is considered “deterministic” if it always
produces the same result for the same input parameters, and
“not deterministic” otherwise. If neither
DETERMINISTIC
nor NOT
DETERMINISTIC
is given in the routine definition, the
default is NOT DETERMINISTIC
. To declare that a
function is deterministic, you must specify
DETERMINISTIC
explicitly.
Assessment of the nature of a routine is based on the
“honesty” of the creator: MySQL does not check that a
routine declared DETERMINISTIC
is free of
statements that produce nondeterministic results. However,
misdeclaring a routine might affect results or affect performance.
Declaring a nondeterministic routine as
DETERMINISTIC
might lead to unexpected results
by causing the optimizer to make incorrect execution plan choices.
Declaring a deterministic routine as
NONDETERMINISTIC
might diminish performance by
causing available optimizations not to be used.
If binary logging is enabled, the DETERMINISTIC
characteristic affects which routine definitions MySQL accepts.
See Section 24.7, “Binary Logging of Stored Programs”.
A routine that contains the NOW()
function (or its synonyms) or
RAND()
is nondeterministic, but it
might still be replication-safe. For
NOW()
, the binary log includes the
timestamp and replicates correctly.
RAND()
also replicates correctly as
long as it is called only a single time during the execution of a
routine. (You can consider the routine execution timestamp and
random number seed as implicit inputs that are identical on the
master and slave.)
Several characteristics provide information about the nature of data use by the routine. In MySQL, these characteristics are advisory only. The server does not use them to constrain what kinds of statements a routine will be permitted to execute.
CONTAINS SQL
indicates that the routine
does not contain statements that read or write data. This is
the default if none of these characteristics is given
explicitly. Examples of such statements are SET @x =
1
or DO RELEASE_LOCK('abc')
,
which execute but neither read nor write data.
NO SQL
indicates that the routine contains
no SQL statements.
READS SQL DATA
indicates that the routine
contains statements that read data (for example,
SELECT
), but not statements
that write data.
MODIFIES SQL DATA
indicates that the
routine contains statements that may write data (for example,
INSERT
or
DELETE
).
The SQL SECURITY
characteristic can be
DEFINER
or INVOKER
to
specify the security context; that is, whether the routine
executes using the privileges of the account named in the routine
DEFINER
clause or the user who invokes it. This
account must have permission to access the database with which the
routine is associated. The default value is
DEFINER
. The user who invokes the routine must
have the EXECUTE
privilege for it,
as must the DEFINER
account if the routine
executes in definer security context.
The DEFINER
clause specifies the MySQL account
to be used when checking access privileges at routine execution
time for routines that have the SQL SECURITY
DEFINER
characteristic.
If a user
value is given for the
DEFINER
clause, it should be a MySQL account
specified as
'
,
user_name
'@'host_name
'CURRENT_USER
, or
CURRENT_USER()
. The default
DEFINER
value is the user who executes the
CREATE PROCEDURE
or
CREATE FUNCTION
statement. This is
the same as specifying DEFINER = CURRENT_USER
explicitly.
If you specify the DEFINER
clause, these rules
determine the valid DEFINER
user values:
If you do not have the
SET_USER_ID
or
SUPER
privilege, the only
permitted user
value is your own
account, either specified literally or by using
CURRENT_USER
. You cannot set
the definer to some other account.
If you have the SET_USER_ID
or
SUPER
privilege, you can
specify any syntactically valid account name. If the account
does not exist, a warning is generated.
Although it is possible to create a routine with a nonexistent
DEFINER
account, an error occurs at routine
execution time if the SQL SECURITY
value is
DEFINER
but the definer account does not
exist.
For more information about stored routine security, see Section 24.6, “Access Control for Stored Programs and Views”.
Within a stored routine that is defined with the SQL
SECURITY DEFINER
characteristic,
CURRENT_USER
returns the routine's
DEFINER
value. For information about user
auditing within stored routines, see
Section 6.3.13, “SQL-Based MySQL Account Activity Auditing”.
Consider the following procedure, which displays a count of the
number of MySQL accounts listed in the
mysql.user
system table:
CREATE DEFINER = 'admin'@'localhost' PROCEDURE account_count() BEGIN SELECT 'Number of accounts:', COUNT(*) FROM mysql.user; END;
The procedure is assigned a DEFINER
account of
'admin'@'localhost'
no matter which user
defines it. It executes with the privileges of that account no
matter which user invokes it (because the default security
characteristic is DEFINER
). The procedure
succeeds or fails depending on whether invoker has the
EXECUTE
privilege for it and
'admin'@'localhost'
has the
SELECT
privilege for the
mysql.user
table.
Now suppose that the procedure is defined with the SQL
SECURITY INVOKER
characteristic:
CREATE DEFINER = 'admin'@'localhost' PROCEDURE account_count() SQL SECURITY INVOKER BEGIN SELECT 'Number of accounts:', COUNT(*) FROM mysql.user; END;
The procedure still has a DEFINER
of
'admin'@'localhost'
, but in this case, it
executes with the privileges of the invoking user. Thus, the
procedure succeeds or fails depending on whether the invoker has
the EXECUTE
privilege for it and
the SELECT
privilege for the
mysql.user
table.
The server handles the data type of a routine parameter, local
routine variable created with
DECLARE
, or function return value
as follows:
Assignments are checked for data type mismatches and overflow. Conversion and overflow problems result in warnings, or errors in strict SQL mode.
Only scalar values can be assigned. For example, a statement
such as SET x = (SELECT 1, 2)
is invalid.
For character data types, if CHARACTER SET
is includedd in the declaration, the specified character set
and its default collation is used. If the
COLLATE
attribute is also present, that
collation is used rather than the default collation.
If CHARACTER SET
and
COLLATE
are not present, the database
character set and collation in effect at routine creation time
are used. To avoid having the server use the database
character set and collation, provide an explicit
CHARACTER SET
and a
COLLATE
attribute for character data
parameters.
If you change the database default character set or collation, stored routines that use the database defaults must be dropped and recreated so that they use the new defaults.
The database character set and collation are given by the
value of the
character_set_database
and
collation_database
system
variables. For more information, see
Section 10.3.3, “Database Character Set and Collation”.
CREATE SERVERserver_name
FOREIGN DATA WRAPPERwrapper_name
OPTIONS (option
[,option
] ...)option
: { HOSTcharacter-literal
| DATABASEcharacter-literal
| USERcharacter-literal
| PASSWORDcharacter-literal
| SOCKETcharacter-literal
| OWNERcharacter-literal
| PORTnumeric-literal
}
This statement creates the definition of a server for use with the
FEDERATED
storage engine. The CREATE
SERVER
statement creates a new row in the
servers
table in the mysql
database. This statement requires the
SUPER
privilege.
The
should be a unique reference to the server. Server definitions are
global within the scope of the server, it is not possible to
qualify the server definition to a specific database.
server_name
has a
maximum length of 64 characters (names longer than 64 characters
are silently truncated), and is case insensitive. You may specify
the name as a quoted string.
server_name
The
is
an identifier and may be quoted with single quotation marks.
wrapper_name
For each
you
must specify either a character literal or numeric literal.
Character literals are UTF-8, support a maximum length of 64
characters and default to a blank (empty) string. String literals
are silently truncated to 64 characters. Numeric literals must be
a number between 0 and 9999, default value is 0.
option
The OWNER
option is currently not applied,
and has no effect on the ownership or operation of the server
connection that is created.
The CREATE SERVER
statement creates an entry in
the mysql.servers
table that can later be used
with the CREATE TABLE
statement
when creating a FEDERATED
table. The options
that you specify will be used to populate the columns in the
mysql.servers
table. The table columns are
Server_name
, Host
,
Db
, Username
,
Password
, Port
and
Socket
.
For example:
CREATE SERVER s FOREIGN DATA WRAPPER mysql OPTIONS (USER 'Remote', HOST '198.51.100.106', DATABASE 'test');
Be sure to specify all options necessary to establish a connection to the server. The user name, host name, and database name are mandatory. Other options might be required as well, such as password.
The data stored in the table can be used when creating a
connection to a FEDERATED
table:
CREATE TABLE t (s1 INT) ENGINE=FEDERATED CONNECTION='s';
For more information, see Section 16.8, “The FEDERATED Storage Engine”.
CREATE SERVER
causes an implicit commit. See
Section 13.3.3, “Statements That Cause an Implicit Commit”.
CREATE SERVER
is not written to the binary log,
regardless of the logging format that is in use.
CREATE OR REPLACE SPATIAL REFERENCE SYSTEMsrid
srs_attribute
... CREATE SPATIAL REFERENCE SYSTEM [IF NOT EXISTS]srid
srs_attribute
...srs_attribute
: { NAME 'srs_name
' | DEFINITION 'definition
' | ORGANIZATION 'org_name
' IDENTIFIED BYorg_id
| DESCRIPTION 'description
' }srid
,org_id
:32-bit unsigned integer
This statement creates a
spatial reference
system (SRS) definition and stores it in the data
dictionary. The definition can be inspected using the
INFORMATION_SCHEMA
ST_SPATIAL_REFERENCE_SYSTEMS
table.
This statement requires the SUPER
privilege.
If neither OR REPLACE
nor IF NOT
EXISTS
is specified, an error occurs if an SRS
definition with the SRID value already exists.
With CREATE OR REPLACE
syntax, any existing SRS
definition with the same SRID value is replaced, unless the SRID
value is used by some column in an existing table. In that case,
an error occurs. For example:
mysql> CREATE OR REPLACE SPATIAL REFERENCE SYSTEM 4326 ...;
ERROR 3716 (SR005): Can't modify SRID 4326. There is at
least one column depending on it.
To identify which column or columns use the SRID, use this query:
SELECT * FROM INFORMATION_SCHEMA.ST_GEOMETRY_COLUMNS WHERE SRS_ID=4326;
With CREATE ... IF NOT EXISTS
syntax, any
existing SRS definition with the same SRID value causes the new
definition to be ignored and a warning occurs.
SRID values must be in the range of 32-bit unsigned integers, with these restrictions:
SRID 0 is a valid SRID but cannot be used with
CREATE SPATIAL REFERENCE
SYSTEM
.
If the value is in a reserved SRID range, a warning occurs. Reserved ranges are [0, 32767] (reserved by EPSG), [60,000,000, 69,999,999] (reserved by EPSG), and [2,000,000,000, 2,147,483,647] (reserved by MySQL). EPSG stands for the European Petroleum Survey Group.
Users should not create SRSs with SRIDs in the reserved ranges. Doing so runs the risk that the SRIDs will conflict with future SRS definitions distributed with MySQL, with the result that the new system-provided SRSs are not installed for MySQL upgrades or that the user-defined SRSs are overwritten.
Attributes for the statement must satisfy these conditions:
Attributes can be given in any order, but no attribute can be given more than once.
The NAME
and DEFINITION
attributes are mandatory.
The NAME
srs_name
attribute value must be
unique. The combination of the ORGANIZATION
org_name
and
org_id
attribute values must be
unique.
The NAME
srs_name
attribute value and
ORGANIZATION
org_name
attribute value cannot be
empty or begin or end with whitespace.
String values in attribute specifications cannot contain control characters, including newline.
The following table shows the maximum lengths for string attribute values.
Table 13.6 CREATE SPATIAL REFERENCE SYSTEM Attribute Lengths
Attribute | Maximum Length (characters) |
---|---|
NAME |
80 |
DEFINITION |
4096 |
ORGANIZATION |
256 |
DESCRIPTION |
2048 |
Here is an example CREATE SPATIAL REFERENCE
SYSTEM
statement. The DEFINITION
value is reformatted across multiple lines for readability. (For
the statement to be legal, the value actually must be given on a
single line.)
CREATE SPATIAL REFERENCE SYSTEM 4120 NAME 'Greek' ORGANIZATION 'EPSG' IDENTIFIED BY 4120 DEFINITION 'GEOGCS["Greek",DATUM["Greek",SPHEROID["Bessel 1841", 6377397.155,299.1528128,AUTHORITY["EPSG","7004"]], AUTHORITY["EPSG","6120"]],PRIMEM["Greenwich",0, AUTHORITY["EPSG","8901"]],UNIT["degree",0.017453292519943278, AUTHORITY["EPSG","9122"]],AXIS["Lat",NORTH],AXIS["Lon",EAST], AUTHORITY["EPSG","4120"]]';
The grammar for SRS definitions is based on the grammar defined in OpenGIS Implementation Specification: Coordinate Transformation Services, Revision 1.00, OGC 01-009, January 12, 2001, Section 7.2. This specification is available at http://www.opengeospatial.org/standards/ct.
MySQL incorporates these changes to the specification:
Only the <horz cs>
production rule is
implemented (that is, geographic and projected SRSs).
There is an optional, nonstandard
<authority>
clause for
<parameter>
. This makes it possible
to recognize projection parameters by authority instead of
name.
SRS definitions may not contain newlines.
CREATE [TEMPORARY] TABLE [IF NOT EXISTS]tbl_name
(create_definition
,...) [table_options
] [partition_options
] CREATE [TEMPORARY] TABLE [IF NOT EXISTS]tbl_name
[(create_definition
,...)] [table_options
] [partition_options
] [IGNORE | REPLACE] [AS]query_expression
CREATE [TEMPORARY] TABLE [IF NOT EXISTS]tbl_name
{ LIKEold_tbl_name
| (LIKEold_tbl_name
) }create_definition
:col_name
column_definition
| {INDEX|KEY} [index_name
] [index_type
] (key_part
,...) [index_option
] ... | {FULLTEXT|SPATIAL} [INDEX|KEY] [index_name
] (key_part
,...) [index_option
] ... | [CONSTRAINT [symbol
]] PRIMARY KEY [index_type
] (key_part
,...) [index_option
] ... | [CONSTRAINT [symbol
]] UNIQUE [INDEX|KEY] [index_name
] [index_type
] (key_part
,...) [index_option
] ... | [CONSTRAINT [symbol
]] FOREIGN KEY [index_name
] (col_name
,...)reference_definition
|check_constraint_definition
column_definition
:data_type
[NOT NULL | NULL] [DEFAULT {literal
| (expr
)} ] [AUTO_INCREMENT] [UNIQUE [KEY]] [[PRIMARY] KEY] [COMMENT 'string
'] [COLLATEcollation_name
] [COLUMN_FORMAT {FIXED|DYNAMIC|DEFAULT}] [STORAGE {DISK|MEMORY}] [reference_definition
] [check_constraint_definition
] |data_type
[COLLATEcollation_name
] [GENERATED ALWAYS] AS (expr
) [VIRTUAL | STORED] [NOT NULL | NULL] [UNIQUE [KEY]] [[PRIMARY] KEY] [COMMENT 'string
'] [reference_definition
] [check_constraint_definition
]data_type
: (see Chapter 11, Data Types)key_part
: {col_name
[(length
)] | (expr
)} [ASC | DESC]index_type
: USING {BTREE | HASH}index_option
: KEY_BLOCK_SIZE [=]value
|index_type
| WITH PARSERparser_name
| COMMENT 'string
' | {VISIBLE | INVISIBLE}check_constraint_definition
: [CONSTRAINT [symbol
]] CHECK (expr
) [[NOT] ENFORCED]reference_definition
: REFERENCEStbl_name
(key_part
,...) [MATCH FULL | MATCH PARTIAL | MATCH SIMPLE] [ON DELETEreference_option
] [ON UPDATEreference_option
]reference_option
: RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULTtable_options
:table_option
[[,]table_option
] ...table_option
: AUTO_INCREMENT [=]value
| AVG_ROW_LENGTH [=]value
| [DEFAULT] CHARACTER SET [=]charset_name
| CHECKSUM [=] {0 | 1} | [DEFAULT] COLLATE [=]collation_name
| COMMENT [=] 'string
' | COMPRESSION [=] {'ZLIB'|'LZ4'|'NONE'} | CONNECTION [=] 'connect_string
' | {DATA|INDEX} DIRECTORY [=] 'absolute path to directory
' | DELAY_KEY_WRITE [=] {0 | 1} | ENCRYPTION [=] {'Y' | 'N'} | ENGINE [=]engine_name
| INSERT_METHOD [=] { NO | FIRST | LAST } | KEY_BLOCK_SIZE [=]value
| MAX_ROWS [=]value
| MIN_ROWS [=]value
| PACK_KEYS [=] {0 | 1 | DEFAULT} | PASSWORD [=] 'string
' | ROW_FORMAT [=] {DEFAULT|DYNAMIC|FIXED|COMPRESSED|REDUNDANT|COMPACT} | STATS_AUTO_RECALC [=] {DEFAULT|0|1} | STATS_PERSISTENT [=] {DEFAULT|0|1} | STATS_SAMPLE_PAGES [=]value
| TABLESPACEtablespace_name
[STORAGE {DISK|MEMORY}] | UNION [=] (tbl_name
[,tbl_name
]...)partition_options
: PARTITION BY { [LINEAR] HASH(expr
) | [LINEAR] KEY [ALGORITHM={1|2}] (column_list
) | RANGE{(expr
) | COLUMNS(column_list
)} | LIST{(expr
) | COLUMNS(column_list
)} } [PARTITIONSnum
] [SUBPARTITION BY { [LINEAR] HASH(expr
) | [LINEAR] KEY [ALGORITHM={1|2}] (column_list
) } [SUBPARTITIONSnum
] ] [(partition_definition
[,partition_definition
] ...)]partition_definition
: PARTITIONpartition_name
[VALUES {LESS THAN {(expr
|value_list
) | MAXVALUE} | IN (value_list
)}] [[STORAGE] ENGINE [=]engine_name
] [COMMENT [=] 'string
' ] [DATA DIRECTORY [=] ''] [INDEX DIRECTORY [=] '
data_dir
'] [MAX_ROWS [=]
index_dir
max_number_of_rows
] [MIN_ROWS [=]min_number_of_rows
] [TABLESPACE [=] tablespace_name] [(subpartition_definition
[,subpartition_definition
] ...)]subpartition_definition
: SUBPARTITIONlogical_name
[[STORAGE] ENGINE [=]engine_name
] [COMMENT [=] 'string
' ] [DATA DIRECTORY [=] ''] [INDEX DIRECTORY [=] '
data_dir
'] [MAX_ROWS [=]
index_dir
max_number_of_rows
] [MIN_ROWS [=]min_number_of_rows
] [TABLESPACE [=] tablespace_name]query_expression:
SELECT ... (Some valid select or union statement
)
CREATE TABLE
creates a table with
the given name. You must have the
CREATE
privilege for the table.
By default, tables are created in the default database, using the
InnoDB
storage engine. An error
occurs if the table exists, if there is no default database, or if
the database does not exist.
For information about the physical representation of a table, see Section 13.1.20.2, “Files Created by CREATE TABLE”.
The original CREATE TABLE
statement, including all specifications and table options are
stored by MySQL when the table is created. For more information,
see Section 13.1.20.1, “CREATE TABLE Statement Retention”.
There are several aspects to the CREATE
TABLE
statement, described under the following topics in
this section:
tbl_name
The table name can be specified as
db_name.tbl_name
to create the
table in a specific database. This works regardless of whether
there is a default database, assuming that the database
exists. If you use quoted identifiers, quote the database and
table names separately. For example, write
`mydb`.`mytbl`
, not
`mydb.mytbl`
.
Rules for permissible table names are given in Section 9.2, “Schema Object Names”.
IF NOT EXISTS
Prevents an error from occurring if the table exists. However,
there is no verification that the existing table has a
structure identical to that indicated by the
CREATE TABLE
statement.
You can use the TEMPORARY
keyword when creating
a table. A TEMPORARY
table is visible only
within the current session, and is dropped automatically when the
session is closed. For more information, see
Section 13.1.20.3, “CREATE TEMPORARY TABLE Syntax”.
LIKE
Use CREATE TABLE ... LIKE
to create an
empty table based on the definition of another table,
including any column attributes and indexes defined in the
original table:
CREATE TABLEnew_tbl
LIKEorig_tbl
;
For more information, see Section 13.1.20.4, “CREATE TABLE ... LIKE Syntax”.
[AS]
query_expression
To create one table from another, add a
SELECT
statement at the end of
the CREATE TABLE
statement:
CREATE TABLEnew_tbl
AS SELECT * FROMorig_tbl
;
For more information, see Section 13.1.20.5, “CREATE TABLE ... SELECT Syntax”.
IGNORE|REPLACE
The IGNORE
and REPLACE
options indicate how to handle rows that duplicate unique key
values when copying a table using a
SELECT
statement.
For more information, see Section 13.1.20.5, “CREATE TABLE ... SELECT Syntax”.
There is a hard limit of 4096 columns per table, but the effective maximum may be less for a given table and depends on the factors discussed in Section C.10.4, “Limits on Table Column Count and Row Size”.
data_type
data_type
represents the data type
in a column definition. For a full description of the syntax
available for specifying column data types, as well as
information about the properties of each type, see
Chapter 11, Data Types.
Some attributes do not apply to all data types.
AUTO_INCREMENT
applies only to integer
and floating-point types. Prior to MySQL 8.0.13,
DEFAULT
does not apply to the
BLOB
,
TEXT
,
GEOMETRY
, and
JSON
types.
Character data types (CHAR
,
VARCHAR
, the
TEXT
types,
ENUM
,
SET
, and any synonyms)
synonyms) can include CHARACTER SET
to
specify the character set for the column.
CHARSET
is a synonym for
CHARACTER SET
. A collation for the
character set can be specified with the
COLLATE
attribute, along with any other
attributes. For details, see Chapter 10, Character Sets, Collations, Unicode.
Example:
CREATE TABLE t (c CHAR(20) CHARACTER SET utf8 COLLATE utf8_bin);
MySQL 8.0 interprets length specifications in
character column definitions in characters. Lengths for
BINARY
and
VARBINARY
are in bytes.
For CHAR
,
VARCHAR
,
BINARY
, and
VARBINARY
columns, indexes
can be created that use only the leading part of column
values, using
syntax to specify an index prefix length.
col_name
(length
)BLOB
and
TEXT
columns also can be
indexed, but a prefix length must be
given. Prefix lengths are given in characters for
nonbinary string types and in bytes for binary string
types. That is, index entries consist of the first
length
characters of each
column value for CHAR
,
VARCHAR
, and
TEXT
columns, and the first
length
bytes of each column
value for BINARY
,
VARBINARY
, and
BLOB
columns. Indexing only
a prefix of column values like this can make the index
file much smaller. For additional information about index
prefixes, see Section 13.1.15, “CREATE INDEX Syntax”.
Only the InnoDB
and
MyISAM
storage engines support indexing
on BLOB
and
TEXT
columns. For example:
CREATE TABLE test (blob_col BLOB, INDEX(blob_col(10)));
If a specified index prefix exceeds the maximum column
data type size, CREATE
TABLE
handles the index as follows:
For a nonunique index, either an error occurs (if strict SQL mode is enabled), or the index length is reduced to lie within the maximum column data type size and a warning is produced (if strict SQL mode is not enabled).
For a unique index, an error occurs regardless of SQL mode because reducing the index length might enable insertion of nonunique entries that do not meet the specified uniqueness requirement.
JSON
columns cannot be
indexed. You can work around this restriction by creating
an index on a generated column that extracts a scalar
value from the JSON
column. See
Indexing a Generated Column to Provide a JSON Column Index, for a
detailed example.
NOT NULL | NULL
If neither NULL
nor NOT
NULL
is specified, the column is treated as though
NULL
had been specified.
In MySQL 8.0, only the InnoDB
,
MyISAM
, and MEMORY
storage engines support indexes on columns that can have
NULL
values. In other cases, you must
declare indexed columns as NOT NULL
or an
error results.
DEFAULT
Specifies a default value for a column. For more information
about default value handling, including the case that a column
definition includes no explicit DEFAULT
value, see Section 11.7, “Data Type Default Values”.
If the NO_ZERO_DATE
or
NO_ZERO_IN_DATE
SQL mode is
enabled and a date-valued default is not correct according to
that mode, CREATE TABLE
produces a warning if strict SQL mode is not enabled and an
error if strict mode is enabled. For example, with
NO_ZERO_IN_DATE
enabled,
c1 DATE DEFAULT '2010-00-00'
produces a
warning.
AUTO_INCREMENT
An integer or floating-point column can have the additional
attribute AUTO_INCREMENT
. When you insert a
value of NULL
(recommended) or
0
into an indexed
AUTO_INCREMENT
column, the column is set to
the next sequence value. Typically this is
, where
value
+1value
is the largest value for the
column currently in the table.
AUTO_INCREMENT
sequences begin with
1
.
To retrieve an AUTO_INCREMENT
value after
inserting a row, use the
LAST_INSERT_ID()
SQL function
or the mysql_insert_id()
C API
function. See Section 12.15, “Information Functions”, and
Section 28.7.7.38, “mysql_insert_id()”.
If the NO_AUTO_VALUE_ON_ZERO
SQL mode is enabled, you can store 0
in
AUTO_INCREMENT
columns as
0
without generating a new sequence value.
See Section 5.1.11, “Server SQL Modes”.
There can be only one AUTO_INCREMENT
column
per table, it must be indexed, and it cannot have a
DEFAULT
value. An
AUTO_INCREMENT
column works properly only
if it contains only positive values. Inserting a negative
number is regarded as inserting a very large positive number.
This is done to avoid precision problems when numbers
“wrap” over from positive to negative and also to
ensure that you do not accidentally get an
AUTO_INCREMENT
column that contains
0
.
For MyISAM
tables, you can specify an
AUTO_INCREMENT
secondary column in a
multiple-column key. See
Section 3.6.9, “Using AUTO_INCREMENT”.
To make MySQL compatible with some ODBC applications, you can
find the AUTO_INCREMENT
value for the last
inserted row with the following query:
SELECT * FROMtbl_name
WHEREauto_col
IS NULL
This method requires that
sql_auto_is_null
variable is
not set to 0. See Section 5.1.8, “Server System Variables”.
For information about InnoDB
and
AUTO_INCREMENT
, see
Section 15.6.1.4, “AUTO_INCREMENT Handling in InnoDB”. For
information about AUTO_INCREMENT
and MySQL
Replication, see
Section 17.4.1.1, “Replication and AUTO_INCREMENT”.
COMMENT
A comment for a column can be specified with the
COMMENT
option, up to 1024 characters long.
The comment is displayed by the SHOW
CREATE TABLE
and
SHOW FULL
COLUMNS
statements.
COLUMN_FORMAT
In NDB Cluster, it is also possible to specify a data storage
format for individual columns of
NDB
tables using
COLUMN_FORMAT
. Permissible column formats
are FIXED
, DYNAMIC
, and
DEFAULT
. FIXED
is used
to specify fixed-width storage, DYNAMIC
permits the column to be variable-width, and
DEFAULT
causes the column to use
fixed-width or variable-width storage as determined by the
column's data type (possibly overridden by a
ROW_FORMAT
specifier).
For NDB
tables, the default value
for COLUMN_FORMAT
is
FIXED
.
COLUMN_FORMAT
currently has no effect on
columns of tables using storage engines other than
NDB
. MySQL 8.0
silently ignores COLUMN_FORMAT
.
STORAGE
For NDB
tables, it is possible to
specify whether the column is stored on disk or in memory by
using a STORAGE
clause. STORAGE
DISK
causes the column to be stored on disk, and
STORAGE MEMORY
causes in-memory storage to
be used. The CREATE TABLE
statement used must still include a
TABLESPACE
clause:
mysql>CREATE TABLE t1 (
->c1 INT STORAGE DISK,
->c2 INT STORAGE MEMORY
->) ENGINE NDB;
ERROR 1005 (HY000): Can't create table 'c.t1' (errno: 140) mysql>CREATE TABLE t1 (
->c1 INT STORAGE DISK,
->c2 INT STORAGE MEMORY
->) TABLESPACE ts_1 ENGINE NDB;
Query OK, 0 rows affected (1.06 sec)
For NDB
tables, STORAGE
DEFAULT
is equivalent to STORAGE
MEMORY
.
The STORAGE
clause has no effect on tables
using storage engines other than
NDB
. The
STORAGE
keyword is supported only in the
build of mysqld that is supplied with NDB
Cluster; it is not recognized in any other version of MySQL,
where any attempt to use the STORAGE
keyword causes a syntax error.
GENERATED ALWAYS
Used to specify a generated column expression. For information about generated columns, see Section 13.1.20.9, “CREATE TABLE and Generated Columns”.
Stored generated
columns can be indexed. InnoDB
supports secondary indexes on
virtual
generated columns. See
Section 13.1.20.10, “Secondary Indexes and Generated Columns”.
Several keywords apply to creation of indexes, foreign keys, and
CHECK
constraints. For general background in
addition to the following descriptions, see
Section 13.1.15, “CREATE INDEX Syntax”,
Section 13.1.20.6, “Using FOREIGN KEY Constraints”, and
Section 13.1.20.7, “CHECK Constraints”.
CONSTRAINT
symbol
The CONSTRAINT
clause may be
given to name a constraint. If the clause is not given, or a
symbol
symbol
is not included following
the CONSTRAINT
keyword, MySQL automatically
generates a constraint name, with the exception noted below.
The symbol
value, if used, must be
unique per schema (database), per constraint type. A duplicate
symbol
results in an error.
If the CONSTRAINT
clause is not
given in a foreign key definition, or a
symbol
symbol
is not included following
the CONSTRAINT
keyword, MySQL uses the
foreign key index name up to MySQL 8.0.15, and automatically
generates a constraint name thereafter.
The SQL standard specifies that all types of constraints (primary key, unique index, foreign key, check) belong to the same namespace. In MySQL, each constraint type has its own namespace per schema. Consequently, names for each type of constraint must be unique per schema.
PRIMARY KEY
A unique index where all key columns must be defined as
NOT NULL
. If they are not explicitly
declared as NOT NULL
, MySQL declares them
so implicitly (and silently). A table can have only one
PRIMARY KEY
. The name of a PRIMARY
KEY
is always PRIMARY
, which thus
cannot be used as the name for any other kind of index.
If you do not have a PRIMARY KEY
and an
application asks for the PRIMARY KEY
in
your tables, MySQL returns the first UNIQUE
index that has no NULL
columns as the
PRIMARY KEY
.
In InnoDB
tables, keep the PRIMARY
KEY
short to minimize storage overhead for secondary
indexes. Each secondary index entry contains a copy of the
primary key columns for the corresponding row. (See
Section 15.6.2.1, “Clustered and Secondary Indexes”.)
In the created table, a PRIMARY KEY
is
placed first, followed by all UNIQUE
indexes, and then the nonunique indexes. This helps the MySQL
optimizer to prioritize which index to use and also more
quickly to detect duplicated UNIQUE
keys.
A PRIMARY KEY
can be a multiple-column
index. However, you cannot create a multiple-column index
using the PRIMARY KEY
key attribute in a
column specification. Doing so only marks that single column
as primary. You must use a separate PRIMARY
KEY(
clause.
key_part
, ...)
If a table has a PRIMARY KEY
or
UNIQUE NOT NULL
index that consists of a
single column that has an integer type, you can use
_rowid
to refer to the indexed column in
SELECT
statements, as described
in Unique Indexes.
In MySQL, the name of a PRIMARY KEY
is
PRIMARY
. For other indexes, if you do not
assign a name, the index is assigned the same name as the
first indexed column, with an optional suffix
(_2
, _3
,
...
) to make it unique. You can see index
names for a table using SHOW INDEX FROM
. See
Section 13.7.6.22, “SHOW INDEX Syntax”.
tbl_name
KEY | INDEX
KEY
is normally a synonym for
INDEX
. The key attribute PRIMARY
KEY
can also be specified as just
KEY
when given in a column definition. This
was implemented for compatibility with other database systems.
UNIQUE
A UNIQUE
index creates a constraint such
that all values in the index must be distinct. An error occurs
if you try to add a new row with a key value that matches an
existing row. For all engines, a UNIQUE
index permits multiple NULL
values for
columns that can contain NULL
. If you
specify a prefix value for a column in a
UNIQUE
index, the column values must be
unique within the prefix length.
If a table has a PRIMARY KEY
or
UNIQUE NOT NULL
index that consists of a
single column that has an integer type, you can use
_rowid
to refer to the indexed column in
SELECT
statements, as described
in Unique Indexes.
FULLTEXT
A FULLTEXT
index is a special type of index
used for full-text searches. Only the
InnoDB
and
MyISAM
storage engines support
FULLTEXT
indexes. They can be created only
from CHAR
,
VARCHAR
, and
TEXT
columns. Indexing always
happens over the entire column; column prefix indexing is not
supported and any prefix length is ignored if specified. See
Section 12.9, “Full-Text Search Functions”, for details of operation. A
WITH PARSER
clause can be specified as an
index_option
value to associate a
parser plugin with the index if full-text indexing and
searching operations need special handling. This clause is
valid only for FULLTEXT
indexes.
InnoDB
and
MyISAM
support full-text parser
plugins. See Full-Text Parser Plugins and
Section 29.2.4.4, “Writing Full-Text Parser Plugins” for more
information.
SPATIAL
You can create SPATIAL
indexes on spatial
data types. Spatial types are supported only for
InnoDB
and MyISAM
tables, and indexed columns must be declared as NOT
NULL
. See Section 11.5, “Spatial Data Types”.
FOREIGN KEY
MySQL supports foreign keys, which let you cross-reference
related data across tables, and foreign key constraints, which
help keep this spread-out data consistent. For definition and
option information, see
reference_definition
,
and
reference_option
.
Partitioned tables employing the
InnoDB
storage engine do not
support foreign keys. See
Section 23.6, “Restrictions and Limitations on Partitioning”, for more
information.
CHECK
The CHECK
clause enables the creation of
constraints to be checked for data values in table rows. See
Section 13.1.20.7, “CHECK Constraints”.
key_part
A key_part
specification can
end with ASC
or DESC
to specify whether index values are stored in ascending or
descending order. The default is ascending if no order
specifier is given.
Prefixes, defined by the length
attribute, can be up to 767 bytes long for
InnoDB
tables that use the
REDUNDANT
or
COMPACT
row format. The prefix length limit is 3072 bytes for
InnoDB
tables that use the
DYNAMIC
or
COMPRESSED
row format. For MyISAM
tables, the
prefix length limit is 1000 bytes.
Prefix limits are measured in bytes.
However, prefix lengths for index
specifications in CREATE
TABLE
, ALTER
TABLE
, and CREATE
INDEX
statements are interpreted as number of
characters for nonbinary string types
(CHAR
,
VARCHAR
,
TEXT
) and number of bytes
for binary string types
(BINARY
,
VARBINARY
,
BLOB
). Take this into
account when specifying a prefix length for a nonbinary
string column that uses a multibyte character set.
index_type
Some storage engines permit you to specify an index type when
creating an index. The syntax for the
index_type
specifier is
USING
.
type_name
Example:
CREATE TABLE lookup (id INT, INDEX USING BTREE (id)) ENGINE = MEMORY;
The preferred position for USING
is after
the index column list. It can be given before the column list,
but support for use of the option in that position is
deprecated and will be removed in a future MySQL release.
index_option
index_option
values specify
additional options for an index.
KEY_BLOCK_SIZE
For MyISAM
tables,
KEY_BLOCK_SIZE
optionally specifies the
size in bytes to use for index key blocks. The value is
treated as a hint; a different size could be used if
necessary. A KEY_BLOCK_SIZE
value
specified for an individual index definition overrides the
table-level KEY_BLOCK_SIZE
value.
For information about the table-level
KEY_BLOCK_SIZE
attribute, see
Table Options.
WITH PARSER
The WITH PARSER
option can only be used
with FULLTEXT
indexes. It associates a
parser plugin with the index if full-text indexing and
searching operations need special handling.
InnoDB
and
MyISAM
support full-text
parser plugins. If you have a
MyISAM
table with an
associated full-text parser plugin, you can convert the
table to InnoDB
using ALTER
TABLE
.
COMMENT
In MySQL 8.0, index definitions can include an optional comment of up to 1024 characters.
You can set the InnoDB
MERGE_THRESHOLD
value for an individual
index using the
index_option
COMMENT
clause. See
Section 15.8.11, “Configuring the Merge Threshold for Index Pages”.
For more information about permissible
index_option
values, see
Section 13.1.15, “CREATE INDEX Syntax”. For more information about
indexes, see Section 8.3.1, “How MySQL Uses Indexes”.
For reference_definition
syntax
details and examples, see
Section 13.1.20.6, “Using FOREIGN KEY Constraints”. For information
specific to foreign keys in InnoDB
, see
Section 15.6.1.5, “InnoDB and FOREIGN KEY Constraints”.
InnoDB
and
NDB
tables support checking of
foreign key constraints. The columns of the referenced table
must always be explicitly named. Both ON
DELETE
and ON UPDATE
actions on
foreign keys are supported. For more detailed information and
examples, see Section 13.1.20.6, “Using FOREIGN KEY Constraints”. For
information specific to foreign keys in
InnoDB
, see
Section 15.6.1.5, “InnoDB and FOREIGN KEY Constraints”.
For other storage engines, MySQL Server parses and ignores the
FOREIGN KEY
and
REFERENCES
syntax in
CREATE TABLE
statements. See
Section 1.8.2.3, “Foreign Key Differences”.
For users familiar with the ANSI/ISO SQL Standard, please
note that no storage engine, including
InnoDB
, recognizes or enforces the
MATCH
clause used in referential
integrity constraint definitions. Use of an explicit
MATCH
clause will not have the specified
effect, and also causes ON DELETE
and
ON UPDATE
clauses to be ignored. For
these reasons, specifying MATCH
should be
avoided.
The MATCH
clause in the SQL standard
controls how NULL
values in a composite
(multiple-column) foreign key are handled when comparing to
a primary key. InnoDB
essentially
implements the semantics defined by MATCH
SIMPLE
, which permit a foreign key to be all or
partially NULL
. In that case, the (child
table) row containing such a foreign key is permitted to be
inserted, and does not match any row in the referenced
(parent) table. It is possible to implement other semantics
using triggers.
Additionally, MySQL requires that the referenced columns be
indexed for performance. However, InnoDB
does not enforce any requirement that the referenced columns
be declared UNIQUE
or NOT
NULL
. The handling of foreign key references to
nonunique keys or keys that contain NULL
values is not well defined for operations such as
UPDATE
or DELETE
CASCADE
. You are advised to use foreign keys that
reference only keys that are both UNIQUE
(or PRIMARY
) and NOT
NULL
.
MySQL parses but ignores “inline
REFERENCES
specifications” (as
defined in the SQL standard) where the references are
defined as part of the column specification. MySQL accepts
REFERENCES
clauses only when specified as
part of a separate FOREIGN KEY
specification.
For information about the RESTRICT
,
CASCADE
, SET NULL
,
NO ACTION
, and SET
DEFAULT
options, see
Section 13.1.20.6, “Using FOREIGN KEY Constraints”.
Table options are used to optimize the behavior of the table. In
most cases, you do not have to specify any of them. These options
apply to all storage engines unless otherwise indicated. Options
that do not apply to a given storage engine may be accepted and
remembered as part of the table definition. Such options then
apply if you later use ALTER TABLE
to convert the table to use a different storage engine.
ENGINE
Specifies the storage engine for the table, using one of the
names shown in the following table. The engine name can be
unquoted or quoted. The quoted name
'DEFAULT'
is recognized but ignored.
Storage Engine | Description |
---|---|
InnoDB |
Transaction-safe tables with row locking and foreign keys. The default
storage engine for new tables. See
Chapter 15, The InnoDB Storage Engine, and in
particular Section 15.1, “Introduction to InnoDB” if you
have MySQL experience but are new to
InnoDB . |
MyISAM |
The binary portable storage engine that is primarily used for read-only or read-mostly workloads. See Section 16.2, “The MyISAM Storage Engine”. |
MEMORY |
The data for this storage engine is stored only in memory. See Section 16.3, “The MEMORY Storage Engine”. |
CSV |
Tables that store rows in comma-separated values format. See Section 16.4, “The CSV Storage Engine”. |
ARCHIVE |
The archiving storage engine. See Section 16.5, “The ARCHIVE Storage Engine”. |
EXAMPLE |
An example engine. See Section 16.9, “The EXAMPLE Storage Engine”. |
FEDERATED |
Storage engine that accesses remote tables. See Section 16.8, “The FEDERATED Storage Engine”. |
HEAP |
This is a synonym for MEMORY . |
MERGE |
A collection of MyISAM tables used as one table. Also
known as MRG_MyISAM . See
Section 16.7, “The MERGE Storage Engine”. |
NDB |
Clustered, fault-tolerant, memory-based tables, supporting transactions
and foreign keys. Also known as
NDBCLUSTER . See
Chapter 22, MySQL NDB Cluster 8.0. |
By default, if a storage engine is specified that is not
available, the statement fails with an error. You can override
this behavior by removing
NO_ENGINE_SUBSTITUTION
from
the server SQL mode (see Section 5.1.11, “Server SQL Modes”) so that
MySQL allows substitution of the specified engine with the
default storage engine instead. Normally in such cases, this
is InnoDB
, which is the default value for
the default_storage_engine
system variable. When
NO_ENGINE_SUBSTITUTION
is disabled, a
warning occurs if the storage engine specification is not
honored.
AUTO_INCREMENT
The initial AUTO_INCREMENT
value for the
table. In MySQL 8.0, this works for
MyISAM
, MEMORY
,
InnoDB
, and ARCHIVE
tables. To set the first auto-increment value for engines that
do not support the AUTO_INCREMENT
table
option, insert a “dummy” row with a value one
less than the desired value after creating the table, and then
delete the dummy row.
For engines that support the AUTO_INCREMENT
table option in CREATE TABLE
statements, you can also use ALTER TABLE
to reset the
tbl_name
AUTO_INCREMENT =
N
AUTO_INCREMENT
value. The value cannot be
set lower than the maximum value currently in the column.
AVG_ROW_LENGTH
An approximation of the average row length for your table. You need to set this only for large tables with variable-size rows.
When you create a MyISAM
table, MySQL uses
the product of the MAX_ROWS
and
AVG_ROW_LENGTH
options to decide how big
the resulting table is. If you don't specify either option,
the maximum size for MyISAM
data and index
files is 256TB by default. (If your operating system does not
support files that large, table sizes are constrained by the
file size limit.) If you want to keep down the pointer sizes
to make the index smaller and faster and you don't really need
big files, you can decrease the default pointer size by
setting the
myisam_data_pointer_size
system variable. (See
Section 5.1.8, “Server System Variables”.) If you want all
your tables to be able to grow above the default limit and are
willing to have your tables slightly slower and larger than
necessary, you can increase the default pointer size by
setting this variable. Setting the value to 7 permits table
sizes up to 65,536TB.
[DEFAULT] CHARACTER SET
Specifies a default character set for the table.
CHARSET
is a synonym for CHARACTER
SET
. If the character set name is
DEFAULT
, the database character set is
used.
CHECKSUM
Set this to 1 if you want MySQL to maintain a live checksum
for all rows (that is, a checksum that MySQL updates
automatically as the table changes). This makes the table a
little slower to update, but also makes it easier to find
corrupted tables. The CHECKSUM
TABLE
statement reports the checksum.
(MyISAM
only.)
[DEFAULT] COLLATE
Specifies a default collation for the table.
COMMENT
A comment for the table, up to 2048 characters long.
You can set the InnoDB
MERGE_THRESHOLD
value for a table using the
table_option
COMMENT
clause. See
Section 15.8.11, “Configuring the Merge Threshold for Index Pages”.
Setting NDB_TABLE options.
The table comment in a CREATE TABLE
that
creates an NDB
table or an
ALTER TABLE
statement which
alters one can also be used to specify one to four of the
NDB_TABLE
options
NOLOGGING
,
READ_BACKUP
,
PARTITION_BALANCE
, or
FULLY_REPLICATED
as a set of name-value
pairs, separated by commas if need be, immediately following
the string NDB_TABLE=
that begins the
quoted comment text. An example statement using this syntax
is shown here (emphasized text):
CREATE TABLE t1 (
c1 INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
c2 VARCHAR(100),
c3 VARCHAR(100) )
ENGINE=NDB
COMMENT="NDB_TABLE=READ_BACKUP=0,PARTITION_BALANCE=FOR_RP_BY_NODE";
Spaces are not permitted within the quoted string. The string is case-insensitive.
The comment is displayed as part of the ouput of
SHOW CREATE TABLE
. The text of
the comment is also available as the TABLE_COMMENT column of
the MySQL Information Schema
TABLES
table.
This comment syntax is also supported with
ALTER TABLE
statements for
NDB
tables. Keep in mind that a table
comment used with ALTER TABLE
replaces any
existing comment which the table might have had perviously.
Setting the MERGE_THRESHOLD
option in table
comments is not supported for NDB
tables (it is ignored).
For complete syntax information and examples, see Section 13.1.20.11, “Setting NDB_TABLE Options”.
COMPRESSION
The compression algorithm used for page level compression for
InnoDB
tables. Supported values include
Zlib
, LZ4
, and
None
. The COMPRESSION
attribute was introduced with the transparent page compression
feature. Page compression is only supported with
InnoDB
tables that reside in
file-per-table
tablespaces, and is only available on Linux and Windows
platforms that support sparse files and hole punching. For
more information, see
Section 15.9.2, “InnoDB Page Compression”.
CONNECTION
The connection string for a FEDERATED
table.
Older versions of MySQL used a COMMENT
option for the connection string.
DATA DIRECTORY
, INDEX
DIRECTORY
For InnoDB
, the DATA
DIRECTORY='
clause permits creating a file-per-table tablespace outside of
the data directory. The tablespace data file is created in the
specified directory, inside a subdirectory with the same name
as the schema. The
directory
'innodb_file_per_table
variable must be enabled to use the DATA
DIRECTORY
clause. The full directory path must be
specified. For more information, see
Section 15.6.3.6, “Creating a Tablespace Outside of the Data Directory”.
When creating MyISAM
tables, you can use
the DATA
DIRECTORY='
clause, the directory
'INDEX
DIRECTORY='
clause, or both. They specify where to put a
directory
'MyISAM
table's data file and index file,
respectively. Unlike InnoDB
tables, MySQL
does not create subdirectories that correspond to the database
name when creating a MyISAM
table with a
DATA DIRECTORY
or INDEX
DIRECTORY
option. Files are created in the directory
that is specified.
You must have the FILE
privilege to use the DATA DIRECTORY
or
INDEX DIRECTORY
table option.
Table-level DATA DIRECTORY
and
INDEX DIRECTORY
options are ignored for
partitioned tables. (Bug #32091)
These options work only when you are not using the
--skip-symbolic-links
option. Your operating system must also have a working,
thread-safe realpath()
call. See
Section 8.12.2.2, “Using Symbolic Links for MyISAM Tables on Unix”, for more complete
information.
If a MyISAM
table is created with no
DATA DIRECTORY
option, the
.MYD
file is created in the database
directory. By default, if MyISAM
finds an
existing .MYD
file in this case, it
overwrites it. The same applies to .MYI
files for tables created with no INDEX
DIRECTORY
option. To suppress this behavior, start
the server with the
--keep_files_on_create
option,
in which case MyISAM
will not overwrite
existing files and returns an error instead.
If a MyISAM
table is created with a
DATA DIRECTORY
or INDEX
DIRECTORY
option and an existing
.MYD
or .MYI
file is
found, MyISAM always returns an error. It will not overwrite a
file in the specified directory.
You cannot use path names that contain the MySQL data
directory with DATA DIRECTORY
or
INDEX DIRECTORY
. This includes
partitioned tables and individual table partitions. (See Bug
#32167.)
DELAY_KEY_WRITE
Set this to 1 if you want to delay key updates for the table
until the table is closed. See the description of the
delay_key_write
system
variable in Section 5.1.8, “Server System Variables”.
(MyISAM
only.)
ENCRYPTION
Set the ENCRYPTION
option to
'Y'
to enable page-level data encryption
for an InnoDB
table created in a
file-per-table
tablespace. Option values are not case-sensitive. The
ENCRYPTION
option was introduced with the
InnoDB
tablespace encryption feature; see
Section 15.6.3.9, “Tablespace Encryption”. A keyring
plugin must be installed and configured before encryption can
be enabled.
INSERT_METHOD
If you want to insert data into a MERGE
table, you must specify with INSERT_METHOD
the table into which the row should be inserted.
INSERT_METHOD
is an option useful for
MERGE
tables only. Use a value of
FIRST
or LAST
to have
inserts go to the first or last table, or a value of
NO
to prevent inserts. See
Section 16.7, “The MERGE Storage Engine”.
KEY_BLOCK_SIZE
For MyISAM
tables,
KEY_BLOCK_SIZE
optionally specifies the
size in bytes to use for index key blocks. The value is
treated as a hint; a different size could be used if
necessary. A KEY_BLOCK_SIZE
value specified
for an individual index definition overrides the table-level
KEY_BLOCK_SIZE
value.
For InnoDB
tables,
KEY_BLOCK_SIZE
specifies the
page size in kilobytes to use
for compressed
InnoDB
tables. The
KEY_BLOCK_SIZE
value is treated as a hint;
a different size could be used by InnoDB
if
necessary. KEY_BLOCK_SIZE
can only be less
than or equal to the
innodb_page_size
value. A
value of 0 represents the default compressed page size, which
is half of the
innodb_page_size
value.
Depending on
innodb_page_size
, possible
KEY_BLOCK_SIZE
values include 0, 1, 2, 4,
8, and 16. See Section 15.9.1, “InnoDB Table Compression” for
more information.
Oracle recommends enabling
innodb_strict_mode
when
specifying KEY_BLOCK_SIZE
for
InnoDB
tables. When
innodb_strict_mode
is
enabled, specifying an invalid
KEY_BLOCK_SIZE
value returns an error. If
innodb_strict_mode
is
disabled, an invalid KEY_BLOCK_SIZE
value
results in a warning, and the
KEY_BLOCK_SIZE
option is ignored.
The Create_options
column in response to
SHOW TABLE STATUS
reports the
actual KEY_BLOCK_SIZE
used by the table, as
does SHOW CREATE TABLE
.
InnoDB
only supports
KEY_BLOCK_SIZE
at the table level.
KEY_BLOCK_SIZE
is not supported with 32KB
and 64KB innodb_page_size
values. InnoDB
table compression does not
support these pages sizes.
InnoDB
does not support the
KEY_BLOCK_SIZE
option when creating
temporary tables.
MAX_ROWS
The maximum number of rows you plan to store in the table. This is not a hard limit, but rather a hint to the storage engine that the table must be able to store at least this many rows.
The use of MAX_ROWS
with
NDB
tables to control the number of table
partitions is deprecated. It remains supported in later
versions for backward compatibility, but is subject to
removal in a future release. Use PARTITION_BALANCE instead;
see
Setting NDB_TABLE options.
The NDB
storage engine treats
this value as a maximum. If you plan to create very large NDB
Cluster tables (containing millions of rows), you should use
this option to insure that NDB
allocates sufficient number of index slots in the hash table
used for storing hashes of the table's primary keys by
setting MAX_ROWS = 2 *
, where
rows
rows
is the number of rows that you
expect to insert into the table.
The maximum MAX_ROWS
value is 4294967295;
larger values are truncated to this limit.
MIN_ROWS
The minimum number of rows you plan to store in the table. The
MEMORY
storage engine uses this
option as a hint about memory use.
PACK_KEYS
Takes effect only with MyISAM
tables. Set
this option to 1 if you want to have smaller indexes. This
usually makes updates slower and reads faster. Setting the
option to 0 disables all packing of keys. Setting it to
DEFAULT
tells the storage engine to pack
only long CHAR
,
VARCHAR
,
BINARY
, or
VARBINARY
columns.
If you do not use PACK_KEYS
, the default is
to pack strings, but not numbers. If you use
PACK_KEYS=1
, numbers are packed as well.
When packing binary number keys, MySQL uses prefix compression:
Every key needs one extra byte to indicate how many bytes of the previous key are the same for the next key.
The pointer to the row is stored in high-byte-first order directly after the key, to improve compression.
This means that if you have many equal keys on two consecutive
rows, all following “same” keys usually only take
two bytes (including the pointer to the row). Compare this to
the ordinary case where the following keys takes
storage_size_for_key + pointer_size
(where
the pointer size is usually 4). Conversely, you get a
significant benefit from prefix compression only if you have
many numbers that are the same. If all keys are totally
different, you use one byte more per key, if the key is not a
key that can have NULL
values. (In this
case, the packed key length is stored in the same byte that is
used to mark if a key is NULL
.)
PASSWORD
This option is unused.
ROW_FORMAT
Defines the physical format in which the rows are stored.
When executing a CREATE TABLE
statement with strict
mode disabled, if you specify a row format that is not
supported by the storage engine that is used for the table,
the table is created using that storage engine's default
row format. The actual row format of the table is reported in
the Row_format
and
Create_options
columns in response to
SHOW TABLE STATUS
.
SHOW CREATE TABLE
also reports
the actual row format of the table.
Row format choices differ depending on the storage engine used for the table.
For InnoDB
tables:
The default row format is defined by
innodb_default_row_format
,
which has a default setting of DYNAMIC
.
The default row format is used when the
ROW_FORMAT
option is not defined or
when ROW_FORMAT=DEFAULT
is used.
If the ROW_FORMAT
option is not
defined, or if ROW_FORMAT=DEFAULT
is
used, operations that rebuild a table also silently change
the row format of the table to the default defined by
innodb_default_row_format
.
For more information, see
Defining the Row Format of a Table.
For more efficient InnoDB
storage of
data types, especially BLOB
types, use the DYNAMIC
. See
DYNAMIC Row Format for
requirements associated with the
DYNAMIC
row format.
To enable compression for InnoDB
tables, specify ROW_FORMAT=COMPRESSED
.
The ROW_FORMAT=COMPRESSED
option is not
supported when creating temporary tables. See
Section 15.9, “InnoDB Table and Page Compression” for requirements
associated with the COMPRESSED
row
format.
The row format used in older versions of MySQL can still
be requested by specifying the
REDUNDANT
row format.
When you specify a non-default
ROW_FORMAT
clause, consider also
enabling the
innodb_strict_mode
configuration option.
ROW_FORMAT=FIXED
is not supported. If
ROW_FORMAT=FIXED
is specified while
innodb_strict_mode
is
disabled, InnoDB
issues a warning and
assumes ROW_FORMAT=DYNAMIC
. If
ROW_FORMAT=FIXED
is specified while
innodb_strict_mode
is
enabled, which is the default, InnoDB
returns an error.
For additional information about InnoDB
row formats, see Section 15.10, “InnoDB Row Formats”.
For MyISAM
tables, the option value can be
FIXED
or DYNAMIC
for
static or variable-length row format.
myisampack sets the type to
COMPRESSED
. See
Section 16.2.3, “MyISAM Table Storage Formats”.
For NDB
tables, the default
ROW_FORMAT
is DYNAMIC
.
STATS_AUTO_RECALC
Specifies whether to automatically recalculate
persistent
statistics for an InnoDB
table. The
value DEFAULT
causes the persistent
statistics setting for the table to be determined by the
innodb_stats_auto_recalc
configuration option. The value 1
causes
statistics to be recalculated when 10% of the data in the
table has changed. The value 0
prevents
automatic recalculation for this table; with this setting,
issue an ANALYZE TABLE
statement to recalculate the statistics after making
substantial changes to the table. For more information about
the persistent statistics feature, see
Section 15.8.10.1, “Configuring Persistent Optimizer Statistics Parameters”.
STATS_PERSISTENT
Specifies whether to enable
persistent
statistics for an InnoDB
table. The
value DEFAULT
causes the persistent
statistics setting for the table to be determined by the
innodb_stats_persistent
configuration option. The value 1
enables
persistent statistics for the table, while the value
0
turns off this feature. After enabling
persistent statistics through a CREATE
TABLE
or ALTER TABLE
statement,
issue an ANALYZE TABLE
statement to calculate the statistics, after loading
representative data into the table. For more information about
the persistent statistics feature, see
Section 15.8.10.1, “Configuring Persistent Optimizer Statistics Parameters”.
STATS_SAMPLE_PAGES
The number of index pages to sample when estimating
cardinality and other statistics for an indexed column, such
as those calculated by ANALYZE
TABLE
. For more information, see
Section 15.8.10.1, “Configuring Persistent Optimizer Statistics Parameters”.
TABLESPACE
The TABLESPACE
clause can be used to create
a table in an existing general tablespace, a file-per-table
tablespace, or the system tablespace.
CREATE TABLEtbl_name
... TABLESPACE [=]tablespace_name
The general tablespace that you specify must exist prior to
using the TABLESPACE
clause. For
information about general tablespaces, see
Section 15.6.3.3, “General Tablespaces”.
The
is a case-sensitive identifier. It may be quoted or unquoted.
The forward slash character (“/”) is not
permitted. Names beginning with “innodb_” are
reserved for special use.
tablespace_name
To create a table in the system tablespace, specify
innodb_system
as the tablespace name.
CREATE TABLE tbl_name
... TABLESPACE [=] innodb_system
Using TABLESPACE [=] innodb_system
, you can
place a table of any uncompressed row format in the system
tablespace regardless of the
innodb_file_per_table
setting. For example, you can add a table with
ROW_FORMAT=DYNAMIC
to the system tablespace
using TABLESPACE [=] innodb_system
.
To create a table in a file-per-table tablespace, specify
innodb_file_per_table
as the tablespace
name.
CREATE TABLE tbl_name
... TABLESPACE [=] innodb_file_per_table
If innodb_file_per_table
is
enabled, you need not specify
TABLESPACE=innodb_file_per_table
to
create an InnoDB
file-per-table
tablespace. InnoDB
tables are created in
file-per-table tablespaces by default when
innodb_file_per_table
is
enabled.
The DATA DIRECTORY
clause is permitted with
CREATE TABLE ...
TABLESPACE=innodb_file_per_table
but is otherwise
not supported for use in combination with the
TABLESPACE
clause.
Support for TABLESPACE =
innodb_file_per_table
and TABLESPACE =
innodb_temporary
clauses with
CREATE
TEMPORARY TABLE
is deprecated as of MySQL 8.0.13
and will be removed in a future version of MySQL.
The STORAGE
table option is employed only
with NDB
tables.
STORAGE
determines the type of storage used
(disk or memory), and can be either DISK
or
MEMORY
.
TABLESPACE ... STORAGE DISK
assigns a table
to an NDB Cluster Disk Data tablespace. The tablespace must
already have been created using CREATE
TABLESPACE
. See
Section 22.5.13, “NDB Cluster Disk Data Tables”, for more
information.
A STORAGE
clause cannot be used in a
CREATE TABLE
statement
without a TABLESPACE
clause.
Used to access a collection of identical
MyISAM
tables as one. This works only with
MERGE
tables. See
Section 16.7, “The MERGE Storage Engine”.
You must have SELECT
,
UPDATE
, and
DELETE
privileges for the
tables you map to a MERGE
table.
Formerly, all tables used had to be in the same database as
the MERGE
table itself. This restriction
no longer applies.
partition_options
can be used to
control partitioning of the table created with
CREATE TABLE
.
Not all options shown in the syntax for
partition_options
at the beginning of
this section are available for all partitioning types. Please see
the listings for the following individual types for information
specific to each type, and see Chapter 23, Partitioning, for
more complete information about the workings of and uses for
partitioning in MySQL, as well as additional examples of table
creation and other statements relating to MySQL partitioning.
Partitions can be modified, merged, added to tables, and dropped from tables. For basic information about the MySQL statements to accomplish these tasks, see Section 13.1.9, “ALTER TABLE Syntax”. For more detailed descriptions and examples, see Section 23.3, “Partition Management”.
PARTITION BY
If used, a partition_options
clause
begins with PARTITION BY
. This clause
contains the function that is used to determine the partition;
the function returns an integer value ranging from 1 to
num
, where
num
is the number of partitions.
(The maximum number of user-defined partitions which a table
may contain is 1024; the number of
subpartitions—discussed later in this section—is
included in this maximum.)
The expression (expr
) used in a
PARTITION BY
clause cannot refer to any
columns not in the table being created; such references are
specifically not permitted and cause the statement to fail
with an error. (Bug #29444)
HASH(
expr
)
Hashes one or more columns to create a key for placing and
locating rows. expr
is an
expression using one or more table columns. This can be any
valid MySQL expression (including MySQL functions) that yields
a single integer value. For example, these are both valid
CREATE TABLE
statements using
PARTITION BY HASH
:
CREATE TABLE t1 (col1 INT, col2 CHAR(5)) PARTITION BY HASH(col1); CREATE TABLE t1 (col1 INT, col2 CHAR(5), col3 DATETIME) PARTITION BY HASH ( YEAR(col3) );
You may not use either VALUES LESS THAN
or
VALUES IN
clauses with PARTITION
BY HASH
.
PARTITION BY HASH
uses the remainder of
expr
divided by the number of
partitions (that is, the modulus). For examples and additional
information, see Section 23.2.4, “HASH Partitioning”.
The LINEAR
keyword entails a somewhat
different algorithm. In this case, the number of the partition
in which a row is stored is calculated as the result of one or
more logical AND
operations. For
discussion and examples of linear hashing, see
Section 23.2.4.1, “LINEAR HASH Partitioning”.
KEY(
column_list
)
This is similar to HASH
, except that MySQL
supplies the hashing function so as to guarantee an even data
distribution. The column_list
argument is simply a list of 1 or more table columns (maximum:
16). This example shows a simple table partitioned by key,
with 4 partitions:
CREATE TABLE tk (col1 INT, col2 CHAR(5), col3 DATE) PARTITION BY KEY(col3) PARTITIONS 4;
For tables that are partitioned by key, you can employ linear
partitioning by using the LINEAR
keyword.
This has the same effect as with tables that are partitioned
by HASH
. That is, the partition number is
found using the
&
operator rather than the modulus (see
Section 23.2.4.1, “LINEAR HASH Partitioning”, and
Section 23.2.5, “KEY Partitioning”, for details). This example
uses linear partitioning by key to distribute data between 5
partitions:
CREATE TABLE tk (col1 INT, col2 CHAR(5), col3 DATE) PARTITION BY LINEAR KEY(col3) PARTITIONS 5;
The ALGORITHM={1|2}
option is supported
with [SUB]PARTITION BY [LINEAR] KEY
.
ALGORITHM=1
causes the server to use the
same key-hashing functions as MySQL 5.1;
ALGORITHM=2
means that the server employs
the key-hashing functions implemented and used by default for
new KEY
partitioned tables in MySQL 5.5 and
later. (Partitioned tables created with the key-hashing
functions employed in MySQL 5.5 and later cannot be used by a
MySQL 5.1 server.) Not specifying the option has the same
effect as using ALGORITHM=2
. This option is
intended for use chiefly when upgrading or downgrading
[LINEAR] KEY
partitioned tables between
MySQL 5.1 and later MySQL versions, or for creating tables
partitioned by KEY
or LINEAR
KEY
on a MySQL 5.5 or later server which can be used
on a MySQL 5.1 server. For more information, see
Section 13.1.9.1, “ALTER TABLE Partition Operations”.
mysqldump in MySQL 5.7 (and later) writes this option encased in versioned comments, like this:
CREATE TABLE t1 (a INT)
/*!50100 PARTITION BY KEY */ /*!50611 ALGORITHM = 1 */ /*!50100 ()
PARTITIONS 3 */
This causes MySQL 5.6.10 and earlier servers to ignore the
option, which would otherwise cause a syntax error in those
versions. If you plan to load a dump made on a MySQL 5.7
server where you use tables that are partitioned or
subpartitioned by KEY
into a MySQL 5.6
server previous to version 5.6.11, be sure to consult
Changes in MySQL 5.6,
before proceeding. (The information found there also applies
if you are loading a dump containing KEY
partitioned or subpartitioned tables made from a MySQL
5.7—actually 5.6.11 or later—server into a MySQL
5.5.30 or earlier server.)
Also in MySQL 5.6.11 and later, ALGORITHM=1
is shown when necessary in the output of
SHOW CREATE TABLE
using
versioned comments in the same manner as
mysqldump. ALGORITHM=2
is always omitted from SHOW CREATE TABLE
output, even if this option was specified when creating the
original table.
You may not use either VALUES LESS THAN
or
VALUES IN
clauses with PARTITION
BY KEY
.
RANGE(
expr
)
In this case, expr
shows a range of
values using a set of VALUES LESS THAN
operators. When using range partitioning, you must define at
least one partition using VALUES LESS THAN
.
You cannot use VALUES IN
with range
partitioning.
For tables partitioned by RANGE
,
VALUES LESS THAN
must be used with either
an integer literal value or an expression that evaluates to
a single integer value. In MySQL 8.0, you can
overcome this limitation in a table that is defined using
PARTITION BY RANGE COLUMNS
, as described
later in this section.
Suppose that you have a table that you wish to partition on a column containing year values, according to the following scheme.
Partition Number: | Years Range: |
---|---|
0 | 1990 and earlier |
1 | 1991 to 1994 |
2 | 1995 to 1998 |
3 | 1999 to 2002 |
4 | 2003 to 2005 |
5 | 2006 and later |
A table implementing such a partitioning scheme can be
realized by the CREATE TABLE
statement shown here:
CREATE TABLE t1 ( year_col INT, some_data INT ) PARTITION BY RANGE (year_col) ( PARTITION p0 VALUES LESS THAN (1991), PARTITION p1 VALUES LESS THAN (1995), PARTITION p2 VALUES LESS THAN (1999), PARTITION p3 VALUES LESS THAN (2002), PARTITION p4 VALUES LESS THAN (2006), PARTITION p5 VALUES LESS THAN MAXVALUE );
PARTITION ... VALUES LESS THAN ...
statements work in a consecutive fashion. VALUES LESS
THAN MAXVALUE
works to specify
“leftover” values that are greater than the
maximum value otherwise specified.
VALUES LESS THAN
clauses work sequentially
in a manner similar to that of the case
portions of a switch ... case
block (as
found in many programming languages such as C, Java, and PHP).
That is, the clauses must be arranged in such a way that the
upper limit specified in each successive VALUES LESS
THAN
is greater than that of the previous one, with
the one referencing MAXVALUE
coming last of
all in the list.
RANGE
COLUMNS(
column_list
)
This variant on RANGE
facilitates partition
pruning for queries using range conditions on multiple columns
(that is, having conditions such as WHERE a = 1 AND b
< 10
or WHERE a = 1 AND b = 10 AND c
< 10
). It enables you to specify value ranges in
multiple columns by using a list of columns in the
COLUMNS
clause and a set of column values
in each PARTITION ... VALUES LESS THAN
(
partition
definition clause. (In the simplest case, this set consists of
a single column.) The maximum number of columns that can be
referenced in the value_list
)column_list
and
value_list
is 16.
The column_list
used in the
COLUMNS
clause may contain only names of
columns; each column in the list must be one of the following
MySQL data types: the integer types; the string types; and
time or date column types. Columns using
BLOB
, TEXT
,
SET
, ENUM
,
BIT
, or spatial data types are not
permitted; columns that use floating-point number types are
also not permitted. You also may not use functions or
arithmetic expressions in the COLUMNS
clause.
The VALUES LESS THAN
clause used in a
partition definition must specify a literal value for each
column that appears in the COLUMNS()
clause; that is, the list of values used for each
VALUES LESS THAN
clause must contain the
same number of values as there are columns listed in the
COLUMNS
clause. An attempt to use more or
fewer values in a VALUES LESS THAN
clause
than there are in the COLUMNS
clause causes
the statement to fail with the error Inconsistency
in usage of column lists for partitioning.... You
cannot use NULL
for any value appearing in
VALUES LESS THAN
. It is possible to use
MAXVALUE
more than once for a given column
other than the first, as shown in this example:
CREATE TABLE rc ( a INT NOT NULL, b INT NOT NULL ) PARTITION BY RANGE COLUMNS(a,b) ( PARTITION p0 VALUES LESS THAN (10,5), PARTITION p1 VALUES LESS THAN (20,10), PARTITION p2 VALUES LESS THAN (50,MAXVALUE), PARTITION p3 VALUES LESS THAN (65,MAXVALUE), PARTITION p4 VALUES LESS THAN (MAXVALUE,MAXVALUE) );
Each value used in a VALUES LESS THAN
value
list must match the type of the corresponding column exactly;
no conversion is made. For example, you cannot use the string
'1'
for a value that matches a column that
uses an integer type (you must use the numeral
1
instead), nor can you use the numeral
1
for a value that matches a column that
uses a string type (in such a case, you must use a quoted
string: '1'
).
For more information, see Section 23.2.1, “RANGE Partitioning”, and Section 23.4, “Partition Pruning”.
LIST(
expr
)
This is useful when assigning partitions based on a table
column with a restricted set of possible values, such as a
state or country code. In such a case, all rows pertaining to
a certain state or country can be assigned to a single
partition, or a partition can be reserved for a certain set of
states or countries. It is similar to
RANGE
, except that only VALUES
IN
may be used to specify permissible values for
each partition.
VALUES IN
is used with a list of values to
be matched. For instance, you could create a partitioning
scheme such as the following:
CREATE TABLE client_firms ( id INT, name VARCHAR(35) ) PARTITION BY LIST (id) ( PARTITION r0 VALUES IN (1, 5, 9, 13, 17, 21), PARTITION r1 VALUES IN (2, 6, 10, 14, 18, 22), PARTITION r2 VALUES IN (3, 7, 11, 15, 19, 23), PARTITION r3 VALUES IN (4, 8, 12, 16, 20, 24) );
When using list partitioning, you must define at least one
partition using VALUES IN
. You cannot use
VALUES LESS THAN
with PARTITION BY
LIST
.
For tables partitioned by LIST
, the value
list used with VALUES IN
must consist of
integer values only. In MySQL 8.0, you can
overcome this limitation using partitioning by LIST
COLUMNS
, which is described later in this section.
LIST
COLUMNS(
column_list
)
This variant on LIST
facilitates partition
pruning for queries using comparison conditions on multiple
columns (that is, having conditions such as WHERE a =
5 AND b = 5
or WHERE a = 1 AND b = 10 AND c
= 5
). It enables you to specify values in multiple
columns by using a list of columns in the
COLUMNS
clause and a set of column values
in each PARTITION ... VALUES IN
(
partition
definition clause.
value_list
)
The rules governing regarding data types for the column list
used in LIST
COLUMNS(
and
the value list used in column_list
)VALUES
IN(
are the
same as those for the column list used in value_list
)RANGE
COLUMNS(
and
the value list used in column_list
)VALUES LESS
THAN(
,
respectively, except that in the value_list
)VALUES IN
clause, MAXVALUE
is not permitted, and you
may use NULL
.
There is one important difference between the list of values
used for VALUES IN
with PARTITION
BY LIST COLUMNS
as opposed to when it is used with
PARTITION BY LIST
. When used with
PARTITION BY LIST COLUMNS
, each element in
the VALUES IN
clause must be a
set of column values; the number of
values in each set must be the same as the number of columns
used in the COLUMNS
clause, and the data
types of these values must match those of the columns (and
occur in the same order). In the simplest case, the set
consists of a single column. The maximum number of columns
that can be used in the column_list
and in the elements making up the
value_list
is 16.
The table defined by the following CREATE
TABLE
statement provides an example of a table using
LIST COLUMNS
partitioning:
CREATE TABLE lc ( a INT NULL, b INT NULL ) PARTITION BY LIST COLUMNS(a,b) ( PARTITION p0 VALUES IN( (0,0), (NULL,NULL) ), PARTITION p1 VALUES IN( (0,1), (0,2), (0,3), (1,1), (1,2) ), PARTITION p2 VALUES IN( (1,0), (2,0), (2,1), (3,0), (3,1) ), PARTITION p3 VALUES IN( (1,3), (2,2), (2,3), (3,2), (3,3) ) );
PARTITIONS
num
The number of partitions may optionally be specified with a
PARTITIONS
clause, where num
num
is the number of
partitions. If both this clause and any
PARTITION
clauses are used,
num
must be equal to the total
number of any partitions that are declared using
PARTITION
clauses.
Whether or not you use a PARTITIONS
clause in creating a table that is partitioned by
RANGE
or LIST
, you
must still include at least one PARTITION
VALUES
clause in the table definition (see below).
SUBPARTITION BY
A partition may optionally be divided into a number of
subpartitions. This can be indicated by using the optional
SUBPARTITION BY
clause. Subpartitioning may
be done by HASH
or KEY
.
Either of these may be LINEAR
. These work
in the same way as previously described for the equivalent
partitioning types. (It is not possible to subpartition by
LIST
or RANGE
.)
The number of subpartitions can be indicated using the
SUBPARTITIONS
keyword followed by an
integer value.
Rigorous checking of the value used in
PARTITIONS
or
SUBPARTITIONS
clauses is applied and this
value must adhere to the following rules:
The value must be a positive, nonzero integer.
No leading zeros are permitted.
The value must be an integer literal, and cannot not be an
expression. For example, PARTITIONS
0.2E+01
is not permitted, even though
0.2E+01
evaluates to
2
. (Bug #15890)
partition_definition
Each partition may be individually defined using a
partition_definition
clause. The
individual parts making up this clause are as follows:
PARTITION
partition_name
Specifies a logical name for the partition.
VALUES
For range partitioning, each partition must include a
VALUES LESS THAN
clause; for list
partitioning, you must specify a VALUES
IN
clause for each partition. This is used to
determine which rows are to be stored in this partition.
See the discussions of partitioning types in
Chapter 23, Partitioning, for syntax examples.
[STORAGE] ENGINE
MySQL accepts a [STORAGE] ENGINE
option
for both PARTITION
and
SUBPARTITION
. Currently, the only way
in which this option can be used is to set all partitions
or all subpartitions to the same storage engine, and an
attempt to set different storage engines for partitions or
subpartitions in the same table will give rise to the
error ERROR 1469 (HY000): The mix of handlers
in the partitions is not permitted in this version of
MySQL.
COMMENT
An optional COMMENT
clause may be used
to specify a string that describes the partition. Example:
COMMENT = 'Data for the years previous to 1999'
The maximum length for a partition comment is 1024 characters.
DATA DIRECTORY
and INDEX
DIRECTORY
DATA DIRECTORY
and INDEX
DIRECTORY
may be used to indicate the directory
where, respectively, the data and indexes for this
partition are to be stored. Both the
and the
data_dir
must be absolute system path names.
index_dir
You must have the FILE
privilege to use the DATA DIRECTORY
or
INDEX DIRECTORY
partition option.
Example:
CREATE TABLE th (id INT, name VARCHAR(30), adate DATE) PARTITION BY LIST(YEAR(adate)) ( PARTITION p1999 VALUES IN (1995, 1999, 2003) DATA DIRECTORY = '/var/appdata/95/data
' INDEX DIRECTORY = '/var/appdata/95/idx
', PARTITION p2000 VALUES IN (1996, 2000, 2004) DATA DIRECTORY = '/var/appdata/96/data
' INDEX DIRECTORY = '/var/appdata/96/idx
', PARTITION p2001 VALUES IN (1997, 2001, 2005) DATA DIRECTORY = '/var/appdata/97/data
' INDEX DIRECTORY = '/var/appdata/97/idx
', PARTITION p2002 VALUES IN (1998, 2002, 2006) DATA DIRECTORY = '/var/appdata/98/data
' INDEX DIRECTORY = '/var/appdata/98/idx
' );
DATA DIRECTORY
and INDEX
DIRECTORY
behave in the same way as in the
CREATE TABLE
statement's table_option
clause as used for MyISAM
tables.
One data directory and one index directory may be specified per partition. If left unspecified, the data and indexes are stored by default in the table's database directory.
The DATA DIRECTORY
and INDEX
DIRECTORY
options are ignored for creating
partitioned tables if
NO_DIR_IN_CREATE
is in
effect.
MAX_ROWS
and
MIN_ROWS
May be used to specify, respectively, the maximum and
minimum number of rows to be stored in the partition. The
values for max_number_of_rows
and min_number_of_rows
must be
positive integers. As with the table-level options with
the same names, these act only as
“suggestions” to the server and are not hard
limits.
TABLESPACE
May be used to designate an InnoDB
file-per-table tablespace for the partition by specifying
TABLESPACE `innodb_file_per_table`
. All
partitions must belong to the same storage engine.
Placing InnoDB
table partitions in
shared InnoDB
tablespaces is not
supported. Shared tablespaces include the
InnoDB
system tablespace and general
tablespaces.
subpartition_definition
The partition definition may optionally contain one or more
subpartition_definition
clauses.
Each of these consists at a minimum of the
SUBPARTITION
, where
name
name
is an identifier for the
subpartition. Except for the replacement of the
PARTITION
keyword with
SUBPARTITION
, the syntax for a subpartition
definition is identical to that for a partition definition.
Subpartitioning must be done by HASH
or
KEY
, and can be done only on
RANGE
or LIST
partitions. See Section 23.2.6, “Subpartitioning”.
Partitioning by Generated Columns
Partitioning by generated columns is permitted. For example:
CREATE TABLE t1 ( s1 INT, s2 INT AS (EXP(s1)) STORED ) PARTITION BY LIST (s2) ( PARTITION p1 VALUES IN (1) );
Partitioning sees a generated column as a regular column, which
enables workarounds for limitations on functions that are not
permitted for partitioning (see
Section 23.6.3, “Partitioning Limitations Relating to Functions”). The
preceding example demonstrates this technique:
EXP()
cannot be used directly in
the PARTITION BY
clause, but a generated column
defined using EXP()
is permitted.
The original CREATE TABLE
statement, including all specifications and table options are
stored by MySQL when the table is created. The information is
retained so that if you change storage engines, collations or
other settings using an ALTER
TABLE
statement, the original table options specified
are retained. This enables you to change between
InnoDB
and
MyISAM
table types even though the
row formats supported by the two engines are different.
Because the text of the original statement is retained, but due
to the way that certain values and options may be silently
reconfigured, the active table definition (accessible through
DESCRIBE
or with
SHOW TABLE STATUS
) and the table
creation string (accessible through SHOW
CREATE TABLE
) may report different values.
For InnoDB
tables, SHOW
CREATE TABLE
and the Create_options
column reported by SHOW TABLE
STATUS
show the actual ROW_FORMAT
and KEY_BLOCK_SIZE
attributes used by the
table. In previous MySQL releases, the originally specified
values for these attributes were reported.
For an InnoDB
table created in a
file-per-table tablespace or general tablespace, table data and
associated indexes are stored in an
ibd file in the database
directory. When an InnoDB
table is created in
the system tablespace, table data and indexes are stored in the
ibdata* files that
represent the system tablespace. The
innodb_file_per_table
option
controls whether tables are created in file-per-table
tablespaces or the system tablespace, by default. The
TABLESPACE
option can be used to place a
table in a file-per-table tablespace, general tablespace, or the
system tablespace, regardless of the
innodb_file_per_table
setting.
For MyISAM
tables, the storage engine creates
data and index files. Thus, for each MyISAM
table tbl_name
, there are two disk
files.
File | Purpose |
---|---|
|
Data file |
|
Index file |
Chapter 16, Alternative Storage Engines, describes what files each storage engine creates to represent tables. If a table name contains special characters, the names for the table files contain encoded versions of those characters as described in Section 9.2.3, “Mapping of Identifiers to File Names”.
You can use the TEMPORARY
keyword when
creating a table. A TEMPORARY
table is
visible only within the current session, and is dropped
automatically when the session is closed. This means that two
different sessions can use the same temporary table name without
conflicting with each other or with an existing
non-TEMPORARY
table of the same name. (The
existing table is hidden until the temporary table is dropped.)
InnoDB
does not support compressed temporary
tables. When innodb_strict_mode
is enabled (the default),
CREATE TEMPORARY
TABLE
returns an error if
ROW_FORMAT=COMPRESSED
or
KEY_BLOCK_SIZE
is specified. If
innodb_strict_mode
is disabled,
warnings are issued and the temporary table is created using a
non-compressed row format. The
innodb_file_per-table
option
does not affect the creation of InnoDB
temporary tables.
CREATE TABLE
causes an implicit
commit, except when used with the TEMPORARY
keyword. See Section 13.3.3, “Statements That Cause an Implicit Commit”.
TEMPORARY
tables have a very loose
relationship with databases (schemas). Dropping a database does
not automatically drop any TEMPORARY
tables
created within that database. Also, you can create a
TEMPORARY
table in a nonexistent database if
you qualify the table name with the database name in the
CREATE TABLE
statement. In this case, all
subsequent references to the table must be qualified with the
database name.
To create a temporary table, you must have the
CREATE TEMPORARY TABLES
privilege. After a session has created a temporary table, the
server performs no further privilege checks on the table. The
creating session can perform any operation on the table, such as
DROP TABLE
,
INSERT
,
UPDATE
, or
SELECT
.
One implication of this behavior is that a session can
manipulate its temporary tables even if the current user has no
privilege to create them. Suppose that the current user does not
have the CREATE TEMPORARY TABLES
privilege but is able to execute a definer-context stored
procedure that executes with the privileges of a user who does
have CREATE TEMPORARY TABLES
and
that creates a temporary table. While the procedure executes,
the session uses the privileges of the defining user. After the
procedure returns, the effective privileges revert to those of
the current user, which can still see the temporary table and
perform any operation on it.
You cannot use CREATE TEMPORY TABLE ... LIKE
to create an empty table based on the definition of a table that
resides in the mysql
tablespace,
InnoDB
system tablespace
(innodb_system
), or a general tablespace. The
tablespace definition for such a table includes a
TABLESPACE
attribute that defines the
tablespace where the table resides, and the aforementioned
tablespaces do not support temporary tables. To create a
temporary table based on the definition of such a table, use
this syntax instead:
CREATE TEMPORARY TABLEnew_tbl
SELECT * FROMorig_tbl
LIMIT 0;
Support for TABLESPACE =
innodb_file_per_table
and TABLESPACE =
innodb_temporary
clauses with
CREATE TEMPORARY
TABLE
is deprecated as of MySQL 8.0.13 and will be
removed in a future version of MySQL.
Use CREATE TABLE ... LIKE
to create an empty
table based on the definition of another table, including any
column attributes and indexes defined in the original table:
CREATE TABLEnew_tbl
LIKEorig_tbl
;
The copy is created using the same version of the table storage
format as the original table. The
SELECT
privilege is required on
the original table.
LIKE
works only for base tables, not for
views.
You cannot execute CREATE TABLE
or
CREATE TABLE ... LIKE
while a
LOCK TABLES
statement is in
effect.
CREATE TABLE ...
LIKE
makes the same checks as
CREATE TABLE
. This means that
if the current SQL mode is different from the mode in effect
when the original table was created, the table definition
might be considered invalid for the new mode and the statement
will fail.
For CREATE TABLE ... LIKE
, the destination
table preserves generated column information from the original
table.
For CREATE TABLE ... LIKE
, the destination
table preserves expression default values from the original
table.
For CREATE TABLE ... LIKE
, the destination
table preserves CHECK
constraints from the
original table, except that all the constraint names are
generated.
CREATE TABLE ... LIKE
does not preserve any
DATA DIRECTORY
or INDEX
DIRECTORY
table options that were specified for the
original table, or any foreign key definitions.
If the original table is a TEMPORARY
table,
CREATE TABLE ... LIKE
does not preserve
TEMPORARY
. To create a
TEMPORARY
destination table, use
CREATE TEMPORARY TABLE ... LIKE
.
Tables created in the mysql
tablespace, the
InnoDB
system tablespace
(innodb_system
), or general tablespaces
include a TABLESPACE
attribute in the table
definition, which defines the tablespace where the table
resides. Due to a temporary regression, CREATE TABLE
... LIKE
preserves the TABLESPACE
attribute and creates the table in the defined tablespace
regardless of the
innodb_file_per_table
setting.
To avoid the TABLESPACE
attribute when
creating an empty table based on the definition of such a table,
use this syntax instead:
CREATE TABLEnew_tbl
SELECT * FROMorig_tbl
LIMIT 0;
You can create one table from another by adding a
SELECT
statement at the end of
the CREATE TABLE
statement:
CREATE TABLEnew_tbl
[AS] SELECT * FROMorig_tbl
;
MySQL creates new columns for all elements in the
SELECT
. For example:
mysql>CREATE TABLE test (a INT NOT NULL AUTO_INCREMENT,
->PRIMARY KEY (a), KEY(b))
->ENGINE=MyISAM SELECT b,c FROM test2;
This creates a MyISAM
table with
three columns, a
, b
, and
c
. The ENGINE
option is
part of the CREATE TABLE
statement, and should not be used following the
SELECT
; this would result in a
syntax error. The same is true for other
CREATE TABLE
options such as
CHARSET
.
Notice that the columns from the
SELECT
statement are appended to
the right side of the table, not overlapped onto it. Take the
following example:
mysql>SELECT * FROM foo;
+---+ | n | +---+ | 1 | +---+ mysql>CREATE TABLE bar (m INT) SELECT n FROM foo;
Query OK, 1 row affected (0.02 sec) Records: 1 Duplicates: 0 Warnings: 0 mysql>SELECT * FROM bar;
+------+---+ | m | n | +------+---+ | NULL | 1 | +------+---+ 1 row in set (0.00 sec)
For each row in table foo
, a row is inserted
in bar
with the values from
foo
and default values for the new columns.
In a table resulting from
CREATE TABLE ...
SELECT
, columns named only in the
CREATE TABLE
part come first.
Columns named in both parts or only in the
SELECT
part come after that. The
data type of SELECT
columns can
be overridden by also specifying the column in the
CREATE TABLE
part.
If any errors occur while copying the data to the table, it is automatically dropped and not created.
You can precede the SELECT
by
IGNORE
or REPLACE
to
indicate how to handle rows that duplicate unique key values.
With IGNORE
, rows that duplicate an existing
row on a unique key value are discarded. With
REPLACE
, new rows replace rows that have the
same unique key value. If neither IGNORE
nor
REPLACE
is specified, duplicate unique key
values result in an error. For more information, see
Comparison of the IGNORE Keyword and Strict SQL Mode.
Because the ordering of the rows in the underlying
SELECT
statements cannot always
be determined, CREATE TABLE ... IGNORE SELECT
and CREATE TABLE ... REPLACE SELECT
statements are flagged as unsafe for statement-based
replication. Such statements produce a warning in the error log
when using statement-based mode and are written to the binary
log using the row-based format when using
MIXED
mode. See also
Section 17.2.1.1, “Advantages and Disadvantages of Statement-Based and Row-Based
Replication”.
CREATE TABLE ...
SELECT
does not automatically create any indexes for
you. This is done intentionally to make the statement as
flexible as possible. If you want to have indexes in the created
table, you should specify these before the
SELECT
statement:
mysql> CREATE TABLE bar (UNIQUE (n)) SELECT n FROM foo;
For CREATE TABLE ... SELECT
, the destination
table does not preserve information about whether columns in the
selected-from table are generated columns. The
SELECT
part of the statement
cannot assign values to generated columns in the destination
table.
For CREATE TABLE ... SELECT
, the destination
table does preserve expression default values from the original
table.
Some conversion of data types might occur. For example, the
AUTO_INCREMENT
attribute is not preserved,
and VARCHAR
columns can become
CHAR
columns. Retrained
attributes are NULL
(or NOT
NULL
) and, for those columns that have them,
CHARACTER SET
, COLLATION
,
COMMENT
, and the DEFAULT
clause.
When creating a table with
CREATE
TABLE ... SELECT
, make sure to alias any function
calls or expressions in the query. If you do not, the
CREATE
statement might fail or result in
undesirable column names.
CREATE TABLE artists_and_works SELECT artist.name, COUNT(work.artist_id) AS number_of_works FROM artist LEFT JOIN work ON artist.id = work.artist_id GROUP BY artist.id;
You can also explicitly specify the data type for a column in the created table:
CREATE TABLE foo (a TINYINT NOT NULL) SELECT b+1 AS a FROM bar;
For CREATE TABLE
... SELECT
, if IF NOT EXISTS
is
given and the target table exists, nothing is inserted into the
destination table, and the statement is not logged.
To ensure that the binary log can be used to re-create the
original tables, MySQL does not permit concurrent inserts during
CREATE TABLE ...
SELECT
.
You cannot use FOR UPDATE
as part of the
SELECT
in a statement such as
CREATE
TABLE
. If you
attempt to do so, the statement fails.
new_table
SELECT ... FROM
old_table
...
MySQL supports foreign keys, which let you cross-reference
related data across tables, and
foreign key
constraints, which help keep this spread-out data
consistent. The essential syntax for a foreign key constraint
definition in a CREATE TABLE
or
ALTER TABLE
statement looks like
this:
[CONSTRAINT [symbol
]] FOREIGN KEY [index_name
] (col_name
, ...) REFERENCEStbl_name
(col_name
,...) [ON DELETEreference_option
] [ON UPDATEreference_option
]reference_option
: RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT
index_name
represents a foreign key
ID. The index_name
value is ignored
if there is already an explicitly defined index on the child
table that can support the foreign key. Otherwise, MySQL
implicitly creates a foreign key index that is named according
to the following rules:
If defined, the CONSTRAINT
symbol
value is used. Otherwise,
the FOREIGN KEY
index_name
value is used.
If neither a CONSTRAINT
symbol
or FOREIGN
KEY
index_name
is
defined, the foreign key index name is generated using the
name of the referencing foreign key column.
Foreign keys definitions are subject to the following conditions:
Foreign key relationships involve a
parent table that
holds the central data values, and a
child table with
identical values pointing back to its parent. The
FOREIGN KEY
clause is specified in the
child table. The parent and child tables must use the same
storage engine. They must not be
TEMPORARY
tables.
In MySQL 8.0, creation of a foreign key
constraint requires the
REFERENCES
privilege for the
parent table.
Corresponding columns in the foreign key and the referenced key must have similar data types. The size and sign of integer types must be the same. The length of string types need not be the same. For nonbinary (character) string columns, the character set and collation must be the same.
When foreign_key_checks
is
enabled, which is the default setting, character set
conversion is not permitted on tables that include a
character string column used in a foreign key constraint.
The workaround is described in
Section 13.1.9, “ALTER TABLE Syntax”.
MySQL requires indexes on foreign keys and referenced keys
so that foreign key checks can be fast and not require a
table scan. In the referencing table, there must be an index
where the foreign key columns are listed as the
first columns in the same order. Such
an index is created on the referencing table automatically
if it does not exist. This index might be silently dropped
later, if you create another index that can be used to
enforce the foreign key constraint.
index_name
, if given, is used as
described previously.
InnoDB
permits a foreign key to reference
any column or group of columns. However, in the referenced
table, there must be an index where the referenced columns
are listed as the first columns in the
same order.
NDB
requires an explicit unique key (or
primary key) on any column referenced as a foreign key.
Index prefixes on foreign key columns are not supported. One
consequence of this is that
BLOB
and
TEXT
columns cannot be
included in a foreign key because indexes on those columns
must always include a prefix length.
If the CONSTRAINT
clause is given,
the symbol
symbol
value, if used, must
be unique in the database. A duplicate
symbol
will result in an error
similar to: ERROR 1022 (2300): Can't write;
duplicate key in table '#sql- 464_1'. If the
clause is not given, or a symbol
is not included following the CONSTRAINT
keyword, MySQL uses the foreign key index name up to MySQL
8.0.15, and automatically generates a constraint name
thereafter.
InnoDB
does not currently
support foreign keys for tables with user-defined
partitioning. This includes both parent and child tables.
This restriction does not apply for
NDB
tables that are partitioned
by KEY
or LINEAR KEY
(the only user partitioning types supported by the
NDB
storage engine); these may have
foreign key references or be the targets of such references.
For NDB
tables, ON
UPDATE CASCADE
is not supported where the
reference is to the parent table's primary key.
Additional aspects of FOREIGN KEY
constraint
usage are described under the following topics in this section:
This section describes how foreign keys help guarantee referential integrity.
For storage engines supporting foreign keys, MySQL rejects any
INSERT
or
UPDATE
operation that attempts
to create a foreign key value in a child table if there is no
a matching candidate key value in the parent table.
When an UPDATE
or
DELETE
operation affects a key
value in the parent table that has matching rows in the child
table, the result depends on the referential
action specified using ON UPDATE
and ON DELETE
subclauses of the
FOREIGN KEY
clause. MySQL supports five
options regarding the action to be taken, listed here:
CASCADE
: Delete or update the row from
the parent table, and automatically delete or update the
matching rows in the child table. Both ON DELETE
CASCADE
and ON UPDATE CASCADE
are supported. Between two tables, do not define several
ON UPDATE CASCADE
clauses that act on
the same column in the parent table or in the child table.
If a FOREIGN KEY
clause is defined on
both tables in a foreign key relationship, making both
tables a parent and child, an ON UPDATE
CASCADE
or ON DELETE CASCADE
subclause defined for one FOREIGN KEY
clause must be defined for the other in order for
cascading operations to succeed. If an ON UPDATE
CASCADE
or ON DELETE CASCADE
subclause is only defined for one FOREIGN
KEY
clause, cascading operations fail with an
error.
Cascaded foreign key actions do not activate triggers.
SET NULL
: Delete or update the row from
the parent table, and set the foreign key column or
columns in the child table to NULL
.
Both ON DELETE SET NULL
and ON
UPDATE SET NULL
clauses are supported.
If you specify a SET NULL
action,
make sure that you have not declared the columns
in the child table as NOT
NULL
.
RESTRICT
: Rejects the delete or update
operation for the parent table. Specifying
RESTRICT
(or NO
ACTION
) is the same as omitting the ON
DELETE
or ON UPDATE
clause.
NO ACTION
: A keyword from standard SQL.
In MySQL, equivalent to RESTRICT
. The
MySQL Server rejects the delete or update operation for
the parent table if there is a related foreign key value
in the referenced table. Some database systems have
deferred checks, and NO ACTION
is a
deferred check. In MySQL, foreign key constraints are
checked immediately, so NO ACTION
is
the same as RESTRICT
.
SET DEFAULT
: This action is recognized
by the MySQL parser, but both
InnoDB
and
NDB
reject table definitions
containing ON DELETE SET DEFAULT
or
ON UPDATE SET DEFAULT
clauses.
For an ON DELETE
or ON
UPDATE
that is not specified, the default action is
always RESTRICT
.
MySQL supports foreign key references between one column and another within a table. (A column cannot have a foreign key reference to itself.) In these cases, “child table records” really refers to dependent records within the same table.
A foreign key constraint on a stored generated column cannot
use ON UPDATE CASCADE
, ON DELETE
SET NULL
, ON UPDATE SET NULL
,
ON DELETE SET DEFAULT
, or ON
UPDATE SET DEFAULT
.
A foreign key constraint cannot reference a virtual generated column.
For InnoDB
restrictions related to foreign
keys and generated columns, see
Section 15.6.1.5, “InnoDB and FOREIGN KEY Constraints”.
Here is a simple example that relates
parent
and child
tables
through a single-column foreign key:
CREATE TABLE parent ( id INT NOT NULL, PRIMARY KEY (id) ) ENGINE=INNODB; CREATE TABLE child ( id INT, parent_id INT, INDEX par_ind (parent_id), FOREIGN KEY (parent_id) REFERENCES parent(id) ON DELETE CASCADE ) ENGINE=INNODB;
A more complex example in which a
product_order
table has foreign keys for
two other tables. One foreign key references a two-column
index in the product
table. The other
references a single-column index in the
customer
table:
CREATE TABLE product ( category INT NOT NULL, id INT NOT NULL, price DECIMAL, PRIMARY KEY(category, id) ) ENGINE=INNODB; CREATE TABLE customer ( id INT NOT NULL, PRIMARY KEY (id) ) ENGINE=INNODB; CREATE TABLE product_order ( no INT NOT NULL AUTO_INCREMENT, product_category INT NOT NULL, product_id INT NOT NULL, customer_id INT NOT NULL, PRIMARY KEY(no), INDEX (product_category, product_id), INDEX (customer_id), FOREIGN KEY (product_category, product_id) REFERENCES product(category, id) ON UPDATE CASCADE ON DELETE RESTRICT, FOREIGN KEY (customer_id) REFERENCES customer(id) ) ENGINE=INNODB;
You can add a new foreign key constraint to an existing table
by using ALTER TABLE
. The
syntax relating to foreign keys for this statement is shown
here:
ALTER TABLEtbl_name
ADD [CONSTRAINT [symbol
]] FOREIGN KEY [index_name
] (col_name
, ...) REFERENCEStbl_name
(col_name
,...) [ON DELETEreference_option
] [ON UPDATEreference_option
]
The foreign key can be self referential (referring to the same
table). When you add a foreign key constraint to a table using
ALTER TABLE
, remember
to create the required indexes first.
You can also use ALTER TABLE
to
drop foreign keys, using the syntax shown here:
ALTER TABLEtbl_name
DROP FOREIGN KEYfk_symbol
;
If the FOREIGN KEY
clause included a
CONSTRAINT
name when you created the
foreign key, you can refer to that name to drop the foreign
key. Otherwise, the fk_symbol
value
is generated internally when the foreign key is created. To
find out the symbol value when you want to drop a foreign key,
use a SHOW CREATE TABLE
statement, as shown here:
mysql>SHOW CREATE TABLE ibtest11c\G
*************************** 1. row *************************** Table: ibtest11c Create Table: CREATE TABLE `ibtest11c` ( `A` int(11) NOT NULL auto_increment, `D` int(11) NOT NULL default '0', `B` varchar(200) NOT NULL default '', `C` varchar(175) default NULL, PRIMARY KEY (`A`,`D`,`B`), KEY `B` (`B`,`C`), KEY `C` (`C`), CONSTRAINT `0_38775` FOREIGN KEY (`A`, `D`) REFERENCES `ibtest11a` (`A`, `D`) ON DELETE CASCADE ON UPDATE CASCADE, CONSTRAINT `0_38776` FOREIGN KEY (`B`, `C`) REFERENCES `ibtest11a` (`B`, `C`) ON DELETE CASCADE ON UPDATE CASCADE ) ENGINE=INNODB CHARSET=utf8mb4 1 row in set (0.01 sec) mysql>ALTER TABLE ibtest11c DROP FOREIGN KEY `0_38775`;
Adding and dropping a foreign key in the same
ALTER TABLE
statement is
supported for
ALTER TABLE ...
ALGORITHM=INPLACE
but is unsupported for
ALTER TABLE ...
ALGORITHM=COPY
.
In MySQL 8.0, the server prohibits changes to
foreign key columns with the potential to cause loss of
referential integrity. A workaround is to use
ALTER TABLE ...
DROP FOREIGN KEY
before changing the column
definition and
ALTER TABLE ...
ADD FOREIGN KEY
afterward.
Table and column identifiers in a FOREIGN KEY ...
REFERENCES ...
clause can be quoted within backticks
(`
). Alternatively, double quotation marks
("
) can be used if the
ANSI_QUOTES
SQL mode is
enabled. The setting of the
lower_case_table_names
system
variable is also taken into account.
You can view a child table's foreign key definitions as
part of the output of the SHOW CREATE
TABLE
statement:
SHOW CREATE TABLE tbl_name
;
You can also obtain information about foreign keys by querying
the
INFORMATION_SCHEMA.KEY_COLUMN_USAGE
table.
You can find information about foreign keys used by
InnoDB
tables in the
INNODB_FOREIGN
and
INNODB_FOREIGN_COLS
tables, also
in the INFORMATION_SCHEMA
database.
mysqldump produces correct definitions of tables in the dump file, including the foreign keys for child tables.
To make it easier to reload dump files for tables that have
foreign key relationships, mysqldump
automatically includes a statement in the dump output to set
foreign_key_checks
to 0. This
avoids problems with tables having to be reloaded in a
particular order when the dump is reloaded. It is also
possible to set this variable manually:
mysql>SET foreign_key_checks = 0;
mysql>SOURCE
mysql>dump_file_name
;SET foreign_key_checks = 1;
This enables you to import the tables in any order if the dump
file contains tables that are not correctly ordered for
foreign keys. It also speeds up the import operation. Setting
foreign_key_checks
to 0 can
also be useful for ignoring foreign key constraints during
LOAD DATA
and
ALTER TABLE
operations.
However, even if foreign_key_checks =
0
, MySQL does not permit the creation of a foreign
key constraint where a column references a nonmatching column
type. Also, if a table has foreign key constraints,
ALTER TABLE
cannot be used to
alter the table to use another storage engine. To change the
storage engine, you must drop any foreign key constraints
first.
You cannot issue DROP TABLE
for
a table that is referenced by a FOREIGN KEY
constraint, unless you do SET foreign_key_checks =
0
. When you drop a table, any constraints that were
defined in the statement used to create that table are also
dropped.
If you re-create a table that was dropped, it must have a
definition that conforms to the foreign key constraints
referencing it. It must have the correct column names and
types, and it must have indexes on the referenced keys, as
stated earlier. If these are not satisfied, MySQL returns
Error 1005 and refers to Error 150 in the error message, which
means that a foreign key constraint was not correctly formed.
Similarly, if an ALTER TABLE
fails due to Error 150, this means that a foreign key
definition would be incorrectly formed for the altered table.
For InnoDB
tables, you can obtain a
detailed explanation of the most recent
InnoDB
foreign key error in the MySQL
Server, by checking the output of
SHOW ENGINE
INNODB STATUS
.
MySQL extends metadata locks, as necessary, to tables that are related by a foreign key constraint. Extending metadata locks prevents conflicting DML and DDL operations from executing concurrently on related tables. This feature also enables updates to foreign key metadata when a parent table is modified. In earlier MySQL releases, foreign key metadata, which is owned by the child table, could not be updated safely.
If a table is locked explicitly with LOCK
TABLES
, any tables related by a foreign key
constraint are opened and locked implicitly. For foreign key
checks, a shared read-only lock
(LOCK TABLES
READ
) is taken on related tables. For cascading
updates, a shared-nothing write lock
(LOCK TABLES
WRITE
) is taken on related tables that are involved
in the operation.
For users familiar with the ANSI/ISO SQL Standard, please note
that no storage engine, including InnoDB
,
recognizes or enforces the MATCH
clause
used in referential-integrity constraint definitions. Use of
an explicit MATCH
clause will not have the
specified effect, and also causes ON DELETE
and ON UPDATE
clauses to be ignored. For
these reasons, specifying MATCH
should be
avoided.
The MATCH
clause in the SQL standard
controls how NULL
values in a composite
(multiple-column) foreign key are handled when comparing to a
primary key. MySQL essentially implements the semantics
defined by MATCH SIMPLE
, which permit a
foreign key to be all or partially NULL
. In
that case, the (child table) row containing such a foreign key
is permitted to be inserted, and does not match any row in the
referenced (parent) table. It is possible to implement other
semantics using triggers.
Additionally, MySQL requires that the referenced columns be
indexed for performance reasons. However, the system does not
enforce a requirement that the referenced columns be
UNIQUE
or be declared NOT
NULL
. The handling of foreign key references to
nonunique keys or keys that contain NULL
values is not well defined for operations such as
UPDATE
or DELETE
CASCADE
. You are advised to use foreign keys that
reference only UNIQUE
(including
PRIMARY
) and NOT NULL
keys.
Furthermore, MySQL parses but ignores “inline
REFERENCES
specifications” (as
defined in the SQL standard) where the references are defined
as part of the column specification. MySQL accepts
REFERENCES
clauses only when specified as
part of a separate FOREIGN KEY
specification. For storage engines that do not support foreign
keys (such as MyISAM
), MySQL
Server parses and ignores foreign key specifications.
The
INFORMATION_SCHEMA.KEY_COLUMN_USAGE
table identifies the key columns that have constraints.
Metadata specific to InnoDB
foreign keys is
found in the INNODB_SYS_FOREIGN
and INNODB_SYS_FOREIGN_COLS
tables.
In the event of a foreign key error involving
InnoDB
tables (usually Error 150 in the
MySQL Server), information about the most recent
InnoDB
foreign key error can be obtained by
checking SHOW
ENGINE INNODB STATUS
output.
If a user has table-level privileges for all parent tables,
ER_NO_REFERENCED_ROW_2
and
ER_ROW_IS_REFERENCED_2
error
messages for foreign key operations expose information about
parent tables. If a user does not have table-level
privileges for all parent tables, more generic error
messages are displayed instead
(ER_NO_REFERENCED_ROW
and
ER_ROW_IS_REFERENCED
).
An exception is that, for stored programs defined to execute
with DEFINER
privileges, the user against
which privileges are assessed is the user in the program
DEFINER
clause, not the invoking user. If
that user has table-level parent table privileges, parent
table information is still displayed. In this case, it is
the responsibility of the stored program creator to hide the
information by including appropriate condition handlers.
Prior to MySQL 8.0.16, CREATE
TABLE
permits only the following limited version of
table CHECK
constraint syntax, which is
parsed and ignored:
CHECK (expr
)
As of MySQL 8.0.16, CREATE TABLE
permits the core features of table and column
CHECK
constraints, for all storage engines.
CREATE TABLE
permits the
following CHECK
constraint syntax, for both
table constraints and column constraints:
[CONSTRAINT [symbol
]] CHECK (expr
) [[NOT] ENFORCED]
The optional symbol
specifies a name
for the constraint. If omitted, MySQL generates a name from the
table name, a literal _chk_
, and an ordinal
number (1, 2, 3, ...). Constraint names have a maximum length of
64 characters. They are case sensitive, but not accent
sensitive.
expr
specifies the constraint
condition as a boolean expression that must evaluate to
TRUE
or UNKNOWN
(for
NULL
values) for each row of the table. If
the condition evaluates to FALSE
, it fails
and a constraint violation occurs. The effect of a violation
depends on the statement being executed, as described later in
this section.
The optional enforcement clause indicates whether the constraint is enforced:
If omitted or specified as ENFORCED
, the
constraint is created and enforced.
If specified as NOT ENFORCED
, the
constraint is created but not enforced.
A CHECK
constraint is specified as either a
table constraint or column constraint:
A table constraint does not appear within a column definition and can refer to any table column or columns. Forward references are permitted to columns appearing later in the table definition.
A column constraint appears within a column definition and can refer only to that column.
Consider this table definition:
CREATE TABLE t1 ( CHECK (c1 <> c2), c1 INT CHECK (c1 > 10), c2 INT CONSTRAINT c2_positive CHECK (c2 > 0), c3 INT CHECK (c3 < 100), CONSTRAINT c1_nonzero CHECK (c1 <> 0), CHECK (c1 > c3) );
The definition includes table constraints and column constraints, in named and unnamed formats:
The first constraint is a table constraint: It occurs outside any column definition, so it can (and does) refer to multiple table columns. This constraint contains forward references to columns not defined yet. No constraint name is specified, so MySQL generates a name.
The next three constraints are column constraints: Each occurs within a column definition, and thus can refer only to the column being defined. One of the constraints is named explicitly. MySQL generates a name for each of the other two.
The last two contraints are table constraints. One of them is named explicitly. MySQL generates a name for the other one.
As mentioned, MySQL generates a name for any
CHECK
constraint specified without one. To
see the names generated for the preceding table definition, use
SHOW CREATE TABLE
:
mysql> SHOW CREATE TABLE t1\G
*************************** 1. row ***************************
Table: t1
Create Table: CREATE TABLE `t1` (
`c1` int(11) DEFAULT NULL,
`c2` int(11) DEFAULT NULL,
`c3` int(11) DEFAULT NULL,
CONSTRAINT `c1_nonzero` CHECK ((`c1` <> 0)),
CONSTRAINT `c2_positive` CHECK ((`c2` > 0)),
CONSTRAINT `t1_chk_1` CHECK ((`c1` <> `c2`)),
CONSTRAINT `t1_chk_2` CHECK ((`c1` > 10)),
CONSTRAINT `t1_chk_3` CHECK ((`c3` < 100)),
CONSTRAINT `t1_chk_4` CHECK ((`c1` > `c3`))
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
The SQL standard specifies that all types of constraints
(primary key, unique index, foreign key, check) belong to the
same namespace. In MySQL, each constraint type has its own
namespace per schema (database). Consequently,
CHECK
constraint names must be unique per
schema; no two tables in the same schema can share a
CHECK
constraint name. (Exception: A
TEMPORARY
table hides a
non-TEMPORARY
table of the same name, so it
can have the same CHECK
constraint names as
well.)
Beginning generated constraint names with the table name helps ensure schema uniqueness because table names also must be unique within the schema.
CHECK
condition expressions must adhere to
the following rules. An error occurs if an expression contains
disallowed constructs.
Nongenerated and generated columns are permitted, except
columns with the AUTO_INCREMENT
attribute
and columns in other tables.
Literals, deterministic built-in functions, and operators
are permitted. A function is deterministic if, given the
same data in tables, multiple invocations produce the same
result, independently of the connected user. Examples of
functions that are nondeterministic and fail this
definition: CONNECTION_ID()
,
CURRENT_USER()
,
NOW()
.
Stored functions and user-defined functions are not permitted.
Stored procedure and function parameters are not permitted.
Variables (system variables, user-defined variables, and stored program local variables) are not permitted.
Subqueries are not permitted.
Foreign key referential actions (ON UPDATE
,
ON DELETE
) are prohibited on columns used in
CHECK
constraints. Likewise,
CHECK
constraints are prohibited on columns
used in foreign key referential actions.
CHECK
constraints are evaluated for
INSERT
,
UPDATE
,
REPLACE
,
LOAD DATA
, and
LOAD XML
statements and an error
occurs if a constraint evaluates to FALSE
. If
an error occurs, handling of changes already applied differs for
transactional and nontransactional storage engines, and also
depends on whether strict SQL mode is in effect, as described in
Strict SQL Mode.
CHECK
constraints are evaluated for
INSERT IGNORE
,
UPDATE IGNORE
,
LOAD DATA ...
IGNORE
, and
LOAD XML ...
IGNORE
statements and a warning occurs if a constraint
evaluates to FALSE
. The insert or update for
any offending row is skipped.
If the constraint expression evaluates to a data type that differs from the declared column type, implicit coercion to the declared type occurs according to the usual MySQL type-conversion rules. See Section 12.2, “Type Conversion in Expression Evaluation”. If type conversion fails or results in a loss of precision, an error occurs.
Constraint expression evaluation uses the SQL mode in effect at evaluation time. If any component of the expression depends on the SQL mode, different results may occur for different uses of the table unless the SQL mode is the same during all uses.
In some cases, MySQL silently changes column specifications from
those given in a CREATE TABLE
or
ALTER TABLE
statement. These
might be changes to a data type, to attributes associated with a
data type, or to an index specification.
All changes are subject to the internal row-size limit of 65,535 bytes, which may cause some attempts at data type changes to fail. See Section C.10.4, “Limits on Table Column Count and Row Size”.
Columns that are part of a PRIMARY KEY
are made NOT NULL
even if not declared
that way.
Trailing spaces are automatically deleted from
ENUM
and
SET
member values when the
table is created.
MySQL maps certain data types used by other SQL database vendors to MySQL types. See Section 11.10, “Using Data Types from Other Database Engines”.
If you include a USING
clause to specify
an index type that is not permitted for a given storage
engine, but there is another index type available that the
engine can use without affecting query results, the engine
uses the available type.
If strict SQL mode is not enabled, a
VARCHAR
column with a length
specification greater than 65535 is converted to
TEXT
, and a
VARBINARY
column with a
length specification greater than 65535 is converted to
BLOB
. Otherwise, an error
occurs in either of these cases.
Specifying the CHARACTER SET binary
attribute for a character data type causes the column to be
created as the corresponding binary data type:
CHAR
becomes
BINARY
,
VARCHAR
becomes
VARBINARY
, and
TEXT
becomes
BLOB
. For the
ENUM
and
SET
data types, this does not
occur; they are created as declared. Suppose that you
specify a table using this definition:
CREATE TABLE t ( c1 VARCHAR(10) CHARACTER SET binary, c2 TEXT CHARACTER SET binary, c3 ENUM('a','b','c') CHARACTER SET binary );
The resulting table has this definition:
CREATE TABLE t ( c1 VARBINARY(10), c2 BLOB, c3 ENUM('a','b','c') CHARACTER SET binary );
To see whether MySQL used a data type other than the one you
specified, issue a DESCRIBE
or
SHOW CREATE TABLE
statement after
creating or altering the table.
Certain other data type changes can occur if you compress a table using myisampack. See Section 16.2.3.3, “Compressed Table Characteristics”.
CREATE TABLE
supports the
specification of generated columns. Values of a generated column
are computed from an expression included in the column
definition.
Generated columns are also supported by the
NDB
storage engine.
The following simple example shows a table that stores the
lengths of the sides of right triangles in the
sidea
and sideb
columns,
and computes the length of the hypotenuse in
sidec
(the square root of the sums of the
squares of the other sides):
CREATE TABLE triangle ( sidea DOUBLE, sideb DOUBLE, sidec DOUBLE AS (SQRT(sidea * sidea + sideb * sideb)) ); INSERT INTO triangle (sidea, sideb) VALUES(1,1),(3,4),(6,8);
Selecting from the table yields this result:
mysql> SELECT * FROM triangle;
+-------+-------+--------------------+
| sidea | sideb | sidec |
+-------+-------+--------------------+
| 1 | 1 | 1.4142135623730951 |
| 3 | 4 | 5 |
| 6 | 8 | 10 |
+-------+-------+--------------------+
Any application that uses the triangle
table
has access to the hypotenuse values without having to specify
the expression that calculates them.
Generated column definitions have this syntax:
col_name
data_type
[GENERATED ALWAYS] AS (expr
) [VIRTUAL | STORED] [NOT NULL | NULL] [UNIQUE [KEY]] [[PRIMARY] KEY] [COMMENT 'string
']
AS (
indicates that the column is generated and defines the
expression used to compute column values. expr
)AS
may be preceded by GENERATED ALWAYS
to make
the generated nature of the column more explicit. Constructs
that are permitted or prohibited in the expression are discussed
later.
The VIRTUAL
or STORED
keyword indicates how column values are stored, which has
implications for column use:
VIRTUAL
: Column values are not stored,
but are evaluated when rows are read, immediately after any
BEFORE
triggers. A virtual column takes
no storage.
InnoDB
supports secondary indexes on
virtual columns. See
Section 13.1.20.10, “Secondary Indexes and Generated Columns”.
STORED
: Column values are evaluated and
stored when rows are inserted or updated. A stored column
does require storage space and can be indexed.
The default is VIRTUAL
if neither keyword is
specified.
It is permitted to mix VIRTUAL
and
STORED
columns within a table.
Other attributes may be given to indicate whether the column is
indexed or can be NULL
, or provide a comment.
Generated column expressions must adhere to the following rules. An error occurs if an expression contains disallowed constructs.
Literals, deterministic built-in functions, and operators
are permitted. A function is deterministic if, given the
same data in tables, multiple invocations produce the same
result, independently of the connected user. Examples of
functions that are nondeterministic and fail this
definition: CONNECTION_ID()
,
CURRENT_USER()
,
NOW()
.
Stored functions and user-defined functions are not permitted.
Stored procedure and function parameters are not permitted.
Variables (system variables, user-defined variables, and stored program local variables) are not permitted.
Subqueries are not permitted.
A generated column definition can refer to other generated columns, but only those occurring earlier in the table definition. A generated column definition can refer to any base (nongenerated) column in the table whether its definition occurs earlier or later.
The AUTO_INCREMENT
attribute cannot be
used in a generated column definition.
An AUTO_INCREMENT
column cannot be used
as a base column in a generated column definition.
If expression evaluation causes truncation or provides
incorrect input to a function, the
CREATE TABLE
statement
terminates with an error and the DDL operation is rejected.
If the expression evaluates to a data type that differs from the declared column type, implicit coercion to the declared type occurs according to the usual MySQL type-conversion rules. See Section 12.2, “Type Conversion in Expression Evaluation”.
Expression evaluation uses the SQL mode in effect at evaluation time. If any component of the expression depends on the SQL mode, different results may occur for different uses of the table unless the SQL mode is the same during all uses.
For CREATE
TABLE ... LIKE
, the destination table preserves
generated column information from the original table.
For CREATE
TABLE ... SELECT
, the destination table does not
preserve information about whether columns in the selected-from
table are generated columns. The
SELECT
part of the statement
cannot assign values to generated columns in the destination
table.
Partitioning by generated columns is permitted. See Table Partitioning.
A foreign key constraint on a stored generated column cannot use
ON UPDATE CASCADE
, ON DELETE SET
NULL
, ON UPDATE SET NULL
,
ON DELETE SET DEFAULT
, or ON UPDATE
SET DEFAULT
.
A foreign key constraint cannot reference a virtual generated column.
For InnoDB
restrictions related to foreign
keys and generated columns, see
Section 15.6.1.5, “InnoDB and FOREIGN KEY Constraints”.
Triggers cannot use
NEW.
or
use col_name
OLD.
to refer to generated columns.
col_name
For INSERT
,
REPLACE
, and
UPDATE
, if a generated column is
inserted into, replaced, or updated explicitly, the only
permitted value is DEFAULT
.
A generated column in a view is considered updatable because it
is possible to assign to it. However, if such a column is
updated explicitly, the only permitted value is
DEFAULT
.
Generated columns have several use cases, such as these:
Virtual generated columns can be used as a way to simplify and unify queries. A complicated condition can be defined as a generated column and referred to from multiple queries on the table to ensure that all of them use exactly the same condition.
Stored generated columns can be used as a materialized cache for complicated conditions that are costly to calculate on the fly.
Generated columns can simulate functional indexes: Use a
generated column to define a functional expression and index
it. This can be useful for working with columns of types
that cannot be indexed directly, such as
JSON
columns; see
Indexing a Generated Column to Provide a JSON Column Index, for a detailed
example.
For stored generated columns, the disadvantage of this approach is that values are stored twice; once as the value of the generated column and once in the index.
If a generated column is indexed, the optimizer recognizes query expressions that match the column definition and uses indexes from the column as appropriate during query execution, even if a query does not refer to the column directly by name. For details, see Section 8.3.11, “Optimizer Use of Generated Column Indexes”.
Example:
Suppose that a table t1
contains
first_name
and last_name
columns and that applications frequently construct the full name
using an expression like this:
SELECT CONCAT(first_name,' ',last_name) AS full_name FROM t1;
One way to avoid writing out the expression is to create a view
v1
on t1
, which simplifies
applications by enabling them to select
full_name
directly without using an
expression:
CREATE VIEW v1 AS SELECT *, CONCAT(first_name,' ',last_name) AS full_name FROM t1; SELECT full_name FROM v1;
A generated column also enables applications to select
full_name
directly without the need to define
a view:
CREATE TABLE t1 ( first_name VARCHAR(10), last_name VARCHAR(10), full_name VARCHAR(255) AS (CONCAT(first_name,' ',last_name)) ); SELECT full_name FROM t1;
InnoDB
supports secondary indexes on virtual
generated columns. Other index types are not supported. A
secondary index defined on a virtual column is sometimes
referred to as a “virtual index”.
A secondary index may be created on one or more virtual columns
or on a combination of virtual columns and regular columns or
stored generated columns. Secondary indexes that include virtual
columns may be defined as UNIQUE
.
When a secondary index is created on a virtual generated column, generated column values are materialized in the records of the index. If the index is a covering index (one that includes all the columns retrieved by a query), generated column values are retrieved from materialized values in the index structure instead of computed “on the fly”.
There are additional write costs to consider when using a
secondary index on a virtual column due to computation performed
when materializing virtual column values in secondary index
records during INSERT
and
UPDATE
operations. Even with
additional write costs, secondary indexes on virtual columns may
be preferable to generated stored columns,
which are materialized in the clustered index, resulting in
larger tables that require more disk space and memory. If a
secondary index is not defined on a virtual column, there are
additional costs for reads, as virtual column values must be
computed each time the column's row is examined.
Values of an indexed virtual column are MVCC-logged to avoid
unnecessary recomputation of generated column values during
rollback or during a purge operation. The data length of logged
values is limited by the index key limit of 767 bytes for
COMPACT
and REDUNDANT
row
formats, and 3072 bytes for DYNAMIC
and
COMPRESSED
row formats.
Adding or dropping a secondary index on a virtual column is an in-place operation.
As noted elsewhere, JSON
columns cannot be indexed directly. To create an index that
references such a column indirectly, you can define a
generated column that extracts the information that should be
indexed, then create an index on the generated column, as
shown in this example:
mysql>CREATE TABLE jemp (
->c JSON,
->g INT GENERATED ALWAYS AS (c->"$.id")),
->INDEX i (g)
->);
Query OK, 0 rows affected (0.28 sec) mysql>INSERT INTO jemp (c) VALUES
>('{"id": "1", "name": "Fred"}'), ('{"id": "2", "name": "Wilma"}'),
>('{"id": "3", "name": "Barney"}'), ('{"id": "4", "name": "Betty"}');
Query OK, 4 rows affected (0.04 sec) Records: 4 Duplicates: 0 Warnings: 0 mysql>SELECT c->>"$.name" AS name
>FROM jemp WHERE g > 2;
+--------+ | name | +--------+ | Barney | | Betty | +--------+ 2 rows in set (0.00 sec) mysql>EXPLAIN SELECT c->>"$.name" AS name
>FROM jemp WHERE g > 2\G
*************************** 1. row *************************** id: 1 select_type: SIMPLE table: jemp partitions: NULL type: range possible_keys: i key: i key_len: 5 ref: NULL rows: 2 filtered: 100.00 Extra: Using where 1 row in set, 1 warning (0.00 sec) mysql>SHOW WARNINGS\G
*************************** 1. row *************************** Level: Note Code: 1003 Message: /* select#1 */ select json_unquote(json_extract(`test`.`jemp`.`c`,'$.name')) AS `name` from `test`.`jemp` where (`test`.`jemp`.`g` > 2) 1 row in set (0.00 sec)
(We have wrapped the output from the last statement in this example to fit the viewing area.)
When you use EXPLAIN
on a
SELECT
or other SQL statement
containing one or more expressions that use the
->
or ->>
operator, these expressions are translated into their
equivalents using JSON_EXTRACT()
and (if
needed) JSON_UNQUOTE()
instead, as shown
here in the output from SHOW
WARNINGS
immediately following this
EXPLAIN
statement:
mysql>EXPLAIN SELECT c->>"$.name"
>FROM jemp WHERE g > 2 ORDER BY c->"$.name"\G
*************************** 1. row *************************** id: 1 select_type: SIMPLE table: jemp partitions: NULL type: range possible_keys: i key: i key_len: 5 ref: NULL rows: 2 filtered: 100.00 Extra: Using where; Using filesort 1 row in set, 1 warning (0.00 sec) mysql>SHOW WARNINGS\G
*************************** 1. row *************************** Level: Note Code: 1003 Message: /* select#1 */ select json_unquote(json_extract(`test`.`jemp`.`c`,'$.name')) AS `c->>"$.name"` from `test`.`jemp` where (`test`.`jemp`.`g` > 2) order by json_extract(`test`.`jemp`.`c`,'$.name') 1 row in set (0.00 sec)
See the descriptions of the
->
and
->>
operators, as well as those of the
JSON_EXTRACT()
and
JSON_UNQUOTE()
functions, for
additional information and examples.
This technique also can be used to provide indexes that
indirectly reference columns of other types that cannot be
indexed directly, such as GEOMETRY
columns.
It is also possible to use indirect indexing of JSON columns in MySQL NDB Cluster, subject to the following conditions:
NDB
handles a
JSON
column value
internally as a BLOB
. This
means that any NDB
table having one or
more JSON columns must have a primary key, else it cannot
be recorded in the binary log.
The NDB
storage engine does
not support indexing of virtual columns. Since the default
for generated columns is VIRTUAL
, you
must specify explicitly the generated column to which to
apply the indirect index as STORED
.
The CREATE TABLE
statement
used to create the table jempn
shown here
is a version of the jemp
table shown
previously, with modifications making it compatible with
NDB
:
CREATE TABLE jempn ( a BIGINT(20) NOT NULL AUTO_INCREMENT PRIMARY KEY, c JSON DEFAULT NULL, g INT GENERATED ALWAYS AS (c->"$.name") STORED, INDEX i (g) ) ENGINE=NDB;
We can populate this table using the following
INSERT
statement:
INSERT INTO jempn (a, c) VALUES (NULL, '{"id": "1", "name": "Fred"}'), (NULL, '{"id": "2", "name": "Wilma"}'), (NULL, '{"id": "3", "name": "Barney"}'), (NULL, '{"id": "4", "name": "Betty"}');
Now NDB
can use index i
,
as shown here:
mysql>EXPLAIN SELECT c->>"$.name" AS name FROM jempn WHERE g > 2\G
*************************** 1. row *************************** id: 1 select_type: SIMPLE table: jempn partitions: p0,p1 type: range possible_keys: i key: i key_len: 5 ref: NULL rows: 3 filtered: 100.00 Extra: Using where with pushed condition (`test`.`jempn`.`g` > 2) 1 row in set, 1 warning (0.00 sec) mysql>SHOW WARNINGS\G
*************************** 1. row *************************** Level: Note Code: 1003 Message: /* select#1 */ select json_unquote(json_extract(`test`.`jempn`.`c`,'$.name')) AS `name` from `test`.`jempn` where (`test`.`jempn`.`g` > 2) 1 row in set (0.00 sec)
You should keep in mind that a stored generated column uses
DataMemory
, and that
an index on such a column uses
IndexMemory
.
In MySQL NDB Cluster, the table comment in a CREATE
TABLE
or ALTER TABLE
statement can also be used to specify an
NDB_TABLE
option, which consists of one or
more name-value pairs, separated by commas if need be, following
the string NDB_TABLE=
. Complete syntax for
names and values syntax is shown here:
COMMENT="NDB_TABLE=ndb_table_option
[,ndb_table_option
[,...]]"ndb_table_option
: NOLOGGING={1|0} | READ_BACKUP={1|0} | PARTITION_BALANCE={FOR_RP_BY_NODE|FOR_RA_BY_NODE|FOR_RP_BY_LDM |FOR_RA_BY_LDM|FOR_RA_BY_LDM_X_2 |FOR_RA_BY_LDM_X_3|FOR_RA_BY_LDM_X_4} | FULLY_REPLICATED={1|0}
Spaces are not permitted within the quoted string. The string is case-insensitive.
The four NDB
table options that can be set as
part of a comment in this way are described in more detail in
the next few paragraphs.
NOLOGGING
: Using 1 corresponds to having
ndb_table_no_logging
enabled,
but has no actual effect. Provided as a placeholder, mostly for
completeness of ALTER TABLE
statements.
READ_BACKUP
: Setting this option to 1 has the
same effect as though
ndb_read_backup
were enabled;
enables reading from any replica. You can set
READ_BACKUP
for an existing table online,
using an ALTER TABLE
statement similar to one
of those shown here:
ALTER TABLE ... ALGORITHM=INPLACE, COMMENT="NDB_TABLE=READ_BACKUP=1"; ALTER TABLE ... ALGORITHM=INPLACE, COMMENT="NDB_TABLE=READ_BACKUP=0";
For more information about the ALGORITHM
option for ALTER TABLE
, see
Section 22.5.14, “Online Operations with ALTER TABLE in NDB Cluster”.
PARTITION_BALANCE
: Provides additional
control over assignment and placement of partitions. The
following four schemes are supported:
FOR_RP_BY_NODE
: One partition per node.
Only one LDM on each node stores a primary partition. Each partition is stored in the same LDM (same ID) on all nodes.
FOR_RA_BY_NODE
: One partition per node
group.
Each node stores a single partition, which can be either a primary replica or a backup replica. Each partition is stored in the same LDM on all nodes.
FOR_RP_BY_LDM
: One partition for each LDM
on each node; the default.
This is the same behavior as prior to MySQL NDB Cluster 7.5.2, except for a slightly different mapping of partitions to LDMs, starting with LDM 0 and placing one partition per node group, then moving on to the next LDM.
This is the setting used if READ_BACKUP
is set to 1.
FOR_RA_BY_LDM
: One partition per LDM in
each node group.
These partitions can be primary or backup partitions.
FOR_RA_BY_LDM_X_2
: Two partitions per LDM
in each node group.
These partitions can be primary or backup partitions.
FOR_RA_BY_LDM_X_3
: Three partitions per
LDM in each node group.
These partitions can be primary or backup partitions.
FOR_RA_BY_LDM_X_4
: Four partitions per
LDM in each node group.
These partitions can be primary or backup partitions.
PARTITION_BALANCE
is the preferred interface
for setting the number of partitions per table. Using
MAX_ROWS
to force the number of partitions is
deprecated but continues to be supported for backward
compatibility; it is subject to removal in a future release of
MySQL NDB Cluster. (Bug #81759, Bug #23544301)
FULLY_REPLICATED
controls whether the table
is fully replicated, that is, whether each data node has a
complete copy of the table. To enable full replication of the
table, use FULLY_REPLICATED=1
.
This setting can also be controlled using the
ndb_fully_replicated
system variable. Setting
it to ON
enables the option by default for
all new NDB
tables; the default is
OFF
. The
ndb_data_node_neighbour
system
variable is also used for fully replicated tables, to ensure
that when a fully replicated table is accessed, we access the
data node which is local to this MySQL Server.
An example of a CREATE TABLE
statement using
such a comment when creating an NDB
table is
shown here:
mysql>CREATE TABLE t1 (
>c1 INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
>c2 VARCHAR(100),
>c3 VARCHAR(100) )
>ENGINE=NDB
>COMMENT="NDB_TABLE=READ_BACKUP=0,PARTITION_BALANCE=FOR_RP_BY_NODE";
The comment is displayed as part of the ouput of
SHOW CREATE TABLE
. The text of
the comment is also available from querying the MySQL
Information Schema TABLES
table, as
in this example:
mysql>SELECT TABLE_NAME, TABLE_SCHEMA, TABLE_COMMENT
>FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME="t1";
+------------+--------------+----------------------------------------------------------+ | TABLE_NAME | TABLE_SCHEMA | TABLE_COMMENT | +------------+--------------+----------------------------------------------------------+ | t1 | c | NDB_TABLE=READ_BACKUP=0,PARTITION_BALANCE=FOR_RP_BY_NODE | | t1 | d | | +------------+--------------+----------------------------------------------------------+ 2 rows in set (0.00 sec)
This comment syntax is also supported with
ALTER TABLE
statements for
NDB
tables. Keep in mind that a table comment
used with ALTER TABLE
replaces any existing
comment which the table might have.
mysql>ALTER TABLE t1 COMMENT="NDB_TABLE=PARTITION_BALANCE=FOR_RA_BY_NODE";
Query OK, 0 rows affected (0.40 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql>SELECT TABLE_NAME, TABLE_SCHEMA, TABLE_COMMENT
>FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME="t1";
+------------+--------------+--------------------------------------------------+ | TABLE_NAME | TABLE_SCHEMA | TABLE_COMMENT | +------------+--------------+--------------------------------------------------+ | t1 | c | NDB_TABLE=PARTITION_BALANCE=FOR_RA_BY_NODE | | t1 | d | | +------------+--------------+--------------------------------------------------+ 2 rows in set (0.01 sec)
You can also see the value of the
PARTITION_BALANCE
option in the output of
ndb_desc. ndb_desc also
shows whether the READ_BACKUP
and
FULLY_REPLICATED
options are set for the
table. See the description of this program for more information.
Because the READ_BACKUP
value was not carried
over to the new comment set by the ALTER
TABLE
statement, there is no longer a way using SQL to
retrieve the value previously set for it. To keep this from
happening, it is suggested that you preserve any such values
from the existing comment string, like this:
mysql>SELECT TABLE_NAME, TABLE_SCHEMA, TABLE_COMMENT
>FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME="t1";
+------------+--------------+----------------------------------------------------------+ | TABLE_NAME | TABLE_SCHEMA | TABLE_COMMENT | +------------+--------------+----------------------------------------------------------+ | t1 | c | NDB_TABLE=READ_BACKUP=0,PARTITION_BALANCE=FOR_RP_BY_NODE | | t1 | d | | +------------+--------------+----------------------------------------------------------+ 2 rows in set (0.00 sec) mysql>ALTER TABLE t1 COMMENT="NDB_TABLE=READ_BACKUP=0,PARTITION_BALANCE=FOR_RA_BY_NODE";
Query OK, 0 rows affected (1.56 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql>SELECT TABLE_NAME, TABLE_SCHEMA, TABLE_COMMENT
>FROM INFORMATION_SCHEMA.TABLES WHERE TABLE_NAME="t1";
+------------+--------------+----------------------------------------------------------------+ | TABLE_NAME | TABLE_SCHEMA | TABLE_COMMENT | +------------+--------------+----------------------------------------------------------------+ | t1 | c | NDB_TABLE=READ_BACKUP=0,PARTITION_BALANCE=FOR_RA_BY_NODE | | t1 | d | | +------------+--------------+----------------------------------------------------------------+ 2 rows in set (0.01 sec)
CREATE [UNDO] TABLESPACEtablespace_name
InnoDB and NDB: [ADD DATAFILE 'file_name
'] InnoDB only: [FILE_BLOCK_SIZE = value] [ENCRYPTION [=] {'Y' | 'N'}] NDB only: USE LOGFILE GROUPlogfile_group
[EXTENT_SIZE [=]extent_size
] [INITIAL_SIZE [=]initial_size
] [AUTOEXTEND_SIZE [=]autoextend_size
] [MAX_SIZE [=]max_size
] [NODEGROUP [=]nodegroup_id
] [WAIT] [COMMENT [=] 'string
'] InnoDB and NDB: [ENGINE [=]engine_name
]
This statement is used to create a tablespace. The precise syntax
and semantics depend on the storage engine used. In standard MySQL
releases, this is always an InnoDB
tablespace. MySQL NDB Cluster also supports tablespaces using the
NDB
storage engine.
CREATE TABLESPACE
syntax is used to
create general tablespaces or undo tablespaces. The
UNDO
keyword, introduced in MySQL 8.0.14, must
be specified to create an undo tablespace.
A general tablespace is a shared tablespace. It can hold multiple tables, and supports all table row formats. General tablespaces can be created in a location relative to or independent of the data directory.
After creating an InnoDB
general tablespace,
use CREATE TABLE
or
tbl_name
... TABLESPACE [=]
tablespace_name
ALTER TABLE
to add tables
to the tablespace. For more information, see
Section 15.6.3.3, “General Tablespaces”.
tbl_name
TABLESPACE [=]
tablespace_name
Undo tablespaces contain undo logs. Undo tablespaces can be created in a chosen location by specifying a fully qualified data file path. For more information, see Section 15.6.3.4, “Undo Tablespaces”.
This statement is used to create a tablespace, which can contain
one or more data files, providing storage space for NDB Cluster
Disk Data tables (see Section 22.5.13, “NDB Cluster Disk Data Tables”).
One data file is created and added to the tablespace using this
statement. Additional data files may be added to the tablespace by
using the ALTER TABLESPACE
statement (see Section 13.1.10, “ALTER TABLESPACE Syntax”).
All NDB Cluster Disk Data objects share the same namespace. This means that each Disk Data object must be uniquely named (and not merely each Disk Data object of a given type). For example, you cannot have a tablespace and a log file group with the same name, or a tablespace and a data file with the same name.
A log file group of one or more UNDO
log files
must be assigned to the tablespace to be created with the
USE LOGFILE GROUP
clause.
logfile_group
must be an existing log
file group created with CREATE LOGFILE
GROUP
(see Section 13.1.16, “CREATE LOGFILE GROUP Syntax”).
Multiple tablespaces may use the same log file group for
UNDO
logging.
When setting EXTENT_SIZE
or
INITIAL_SIZE
, you may optionally follow the
number with a one-letter abbreviation for an order of magnitude,
similar to those used in my.cnf
. Generally,
this is one of the letters M
(for megabytes) or
G
(for gigabytes).
INITIAL_SIZE
and EXTENT_SIZE
are subject to rounding as follows:
EXTENT_SIZE
is rounded up to the nearest
whole multiple of 32K.
INITIAL_SIZE
is rounded
down to the nearest whole multiple of
32K; this result is rounded up to the nearest whole multiple
of EXTENT_SIZE
(after any rounding).
The rounding just described is done explicitly, and a warning is
issued by the MySQL Server when any such rounding is performed.
The rounded values are also used by the NDB kernel for calculating
INFORMATION_SCHEMA.FILES
column
values and other purposes. However, to avoid an unexpected result,
we suggest that you always use whole multiples of 32K in
specifying these options.
When CREATE TABLESPACE
is used with
ENGINE [=] NDB
, a tablespace and associated
data file are created on each Cluster data node. You can verify
that the data files were created and obtain information about them
by querying the
INFORMATION_SCHEMA.FILES
table. (See
the example later in this section.)
(See Section 25.11, “The INFORMATION_SCHEMA FILES Table”.)
ADD DATAFILE
: Defines the name of a
tablespace data file. The ADD DATAFILE
clause is required when creating undo tablespaces. Otherwise,
it is optional as of MySQL 8.0.14.
An InnoDB
tablespace supports only a single
data file, whose name must include a .ibd
extension. An NDB Cluster tablespace supports multiple data
files which can have any legal file names; more data files can
be added to an NDB Cluster tablespace following its creation
by using an ALTER TABLESPACE
statement.
To place a general tablespace data file in a location outside
of the data directory, include a fully qualified path or a
path relative to the data directory. Only a fully qualified
path is permitted for undo tablespaces. If you do not specify
a path, a general tablespace is created in the data directory.
An undo tablespace created without specifying a path is
created in the directory defined by the
innodb_undo_directory
variable. If the
innodb_undo_directory
variable is undefined, undo tablespaces are created in the
data directory.
Creating a general tablespace in a subdirectory under the data
directory is not supported to avoid conflicts with implicitly
created file-per-table tablespaces. When creating a general
tablespace or undo tablespace outside of the data directory,
the directory must exist and must be known to
InnoDB
prior to creating the tablespace. To
make a directory known to InnoDB
, add it to
the innodb_directories
value
or to one of the variables whose values are appended to the
innodb_directories
value.
innodb_directories
is a
read-only variable. Configuring it requires restarting the
server.
The
,
including any specified path, must be quoted with single or
double quotations marks. File names (not counting the file
extension) and directory names must be at least one byte in
length. Zero length file names and directory names are not
supported.
file_name
If the ADD DATAFILE
clause is not specified
when creating a tablespace, a tablespace data file with a
unique file name is created implicitly. The unique file name
is a 128 bit UUID formatted into five groups of hexadecimal
numbers separated by dashes
(aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee
).
A file extension is added if required by the storage engine.
An .ibd
file extension is added for
InnoDB
general tablespace data files. In a
replication environment, the data file name created on the
master is not the same as the data file name created on the
slave.
FILE_BLOCK_SIZE
: This option—which is
specific to InnoDB
general tablespaces, and
is ignored by NDB
—defines the block
size for the tablespace data file. Values can be specified in
bytes or kilobytes. For example, an 8 kilobyte file block size
can be specified as 8192 or 8K. If you do not specify this
option, FILE_BLOCK_SIZE
defaults to the
innodb_page_size
value.
FILE_BLOCK_SIZE
is required when you intend
to use the tablespace for storing compressed
InnoDB
tables
(ROW_FORMAT=COMPRESSED
). In this case, you
must define the tablespace FILE_BLOCK_SIZE
when creating the tablespace.
If FILE_BLOCK_SIZE
is equal the
innodb_page_size
value, the
tablespace can contain only tables having an uncompressed row
format (COMPACT
,
REDUNDANT
, and DYNAMIC
).
Tables with a COMPRESSED
row format have a
different physical page size than uncompressed tables.
Therefore, compressed tables cannot coexist in the same
tablespace as uncompressed tables.
For a general tablespace to contain compressed tables,
FILE_BLOCK_SIZE
must be specified, and the
FILE_BLOCK_SIZE
value must be a valid
compressed page size in relation to the
innodb_page_size
value. Also,
the physical page size of the compressed table
(KEY_BLOCK_SIZE
) must be equal to
FILE_BLOCK_SIZE/1024
. For example, if
innodb_page_size=16K
, and
FILE_BLOCK_SIZE=8K
, the
KEY_BLOCK_SIZE
of the table must be 8. For
more information, see Section 15.6.3.3, “General Tablespaces”.
USE LOGFILE GROUP
: Required for
NDB
, this is the name of a log file group
previously created using CREATE LOGFILE
GROUP
. Not supported for InnoDB
,
where it fails with an error.
EXTENT_SIZE
: This option is specific to
NDB, and is not supported by InnoDB, where it fails with an
error. EXTENT_SIZE
sets the size, in bytes,
of the extents used by any files belonging to the tablespace.
The default value is 1M. The minimum size is 32K, and
theoretical maximum is 2G, although the practical maximum size
depends on a number of factors. In most cases, changing the
extent size does not have any measurable effect on
performance, and the default value is recommended for all but
the most unusual situations.
An extent is a unit of
disk space allocation. One extent is filled with as much data
as that extent can contain before another extent is used. In
theory, up to 65,535 (64K) extents may used per data file;
however, the recommended maximum is 32,768 (32K). The
recommended maximum size for a single data file is
32G—that is, 32K extents × 1 MB per extent. In
addition, once an extent is allocated to a given partition, it
cannot be used to store data from a different partition; an
extent cannot store data from more than one partition. This
means, for example that a tablespace having a single datafile
whose INITIAL_SIZE
(described in the
following item) is 256 MB and whose
EXTENT_SIZE
is 128M has just two extents,
and so can be used to store data from at most two different
disk data table partitions.
You can see how many extents remain free in a given data file
by querying the
INFORMATION_SCHEMA.FILES
table,
and so derive an estimate for how much space remains free in
the file. For further discussion and examples, see
Section 25.11, “The INFORMATION_SCHEMA FILES Table”.
INITIAL_SIZE
: This option is specific to
NDB
, and is not supported by
InnoDB
, where it fails with an error.
The INITIAL_SIZE
parameter sets the total
size in bytes of the data file that was specific using
ADD DATATFILE
. Once this file has been
created, its size cannot be changed; however, you can add more
data files to the tablespace using
ALTER
TABLESPACE ... ADD DATAFILE
.
INITIAL_SIZE
is optional; its default value
is 134217728 (128 MB).
On 32-bit systems, the maximum supported value for
INITIAL_SIZE
is 4294967296 (4 GB).
AUTOEXTEND_SIZE
: Currently ignored by
MySQL; reserved for possible future use. Has no effect in any
release of MySQL 8.0 or MySQL NDB Cluster 8.0, regardless of
the storage engine used.
MAX_SIZE
: Currently ignored by MySQL;
reserved for possible future use. Has no effect in any release
of MySQL 8.0 or MySQL NDB Cluster 8.0, regardless of the
storage engine used.
NODEGROUP
: Currently ignored by MySQL;
reserved for possible future use. Has no effect in any release
of MySQL 8.0 or MySQL NDB Cluster 8.0, regardless of the
storage engine used.
WAIT
: Currently ignored by MySQL; reserved
for possible future use. Has no effect in any release of MySQL
8.0 or MySQL NDB Cluster 8.0, regardless of the storage engine
used.
COMMENT
: Currently ignored by MySQL;
reserved for possible future use. Has no effect in any release
of MySQL 8.0 or MySQL NDB Cluster 8.0, regardless of the
storage engine used.
The ENCRYPTION
option enables or disables
page-level data encryption for an InnoDB
general tablespace. Option values are not case-sensitive.
Encryption support for general tablespaces was introduced in
MySQL 8.0.13.
As of MySQL 8.0.16, if the ENCRYPTION
option is not specified, the
default_table_encryption
variable controls whether encryption is enabled. The
ENCRYPTION
option overrides the
default_table_encryption
setting. However, if the
table_encryption_privilege_check
variable is enabled, the
TABLE_ENCRYPTION_ADMIN
privilege is required to use an ENCRYPTION
option that differs from the
default_table_encryption
setting.
A keyring plugin must be installed and configured before an encryption-enabled tablespace can be created.
When a general tablespace is encrypted, all tables residing in the tablespace are encrypted. Likewise, a table created in an encrypted tablespace is encrypted.
For more information, see Section 15.6.3.9, “Tablespace Encryption”
ENGINE
: Defines the storage engine which
uses the tablespace, where
engine_name
is the name of the
storage engine. Currently, only the InnoDB
storage engine is supported by standard MySQL 8.0
releases. MySQL NDB Cluster supports both
NDB
and InnoDB
tablespaces. The value of the
default_storage_engine
system
variable is used for ENGINE
if the option
is not specified.
For the rules covering the naming of MySQL tablespaces, see
Section 9.2, “Schema Object Names”. In addition to these rules, the
slash character (“/”) is not permitted, nor can
you use names beginning with innodb_
, as
this prefix is reserved for system use.
Creation of temporary general tablespaces is not supported.
General tablespaces do not support temporary tables.
The TABLESPACE
option may be used with
CREATE TABLE
or
ALTER TABLE
to assign an
InnoDB
table partition or subpartition to a
file-per-table tablespace. All partitions must belong to the
same storage engine. Assigning table partitions to shared
InnoDB
tablespaces is not supported. Shared
tablespaces include the InnoDB
system
tablespace and general tablespaces.
General tablespaces support the addition of tables of any row
format using
CREATE TABLE ...
TABLESPACE
.
innodb_file_per_table
does
not need to be enabled.
innodb_strict_mode
is not
applicable to general tablespaces. Tablespace management rules
are strictly enforced independently of
innodb_strict_mode
. If
CREATE TABLESPACE
parameters are incorrect
or incompatible, the operation fails regardless of the
innodb_strict_mode
setting.
When a table is added to a general tablespace using
CREATE TABLE ...
TABLESPACE
or
ALTER TABLE ...
TABLESPACE
,
innodb_strict_mode
is ignored
but the statement is evaluated as if
innodb_strict_mode
is
enabled.
Use DROP TABLESPACE
to remove a tablespace.
All tables must be dropped from a tablespace using
DROP TABLE
prior to dropping
the tablespace. Before dropping an NDB Cluster tablespace you
must also remove all its data files using one or more
ALTER
TABLESPACE ... DROP DATATFILE
statements. See
Section 22.5.13.1, “NDB Cluster Disk Data Objects”.
All parts of an InnoDB
table added to an
InnoDB
general tablespace reside in the
general tablespace, including indexes and
BLOB
pages.
For an NDB
table assigned to a tablespace,
only those columns which are not indexed are stored on disk,
and actually use the tablespace data files. Indexes and
indexed columns for all NDB
tables are
always kept in memory.
Similar to the system tablespace, truncating or dropping
tables stored in a general tablespace creates free space
internally in the general tablespace
.ibd data file which can
only be used for new InnoDB
data. Space is
not released back to the operating system as it is for
file-per-table tablespaces.
A general tablespace is not associated with any database or schema.
ALTER TABLE ...
DISCARD TABLESPACE
and
ALTER TABLE
...IMPORT TABLESPACE
are not supported for tables
that belong to a general tablespace.
The server uses tablespace-level metadata locking for DDL that references general tablespaces. By comparison, the server uses table-level metadata locking for DDL that references file-per-table tablespaces.
A generated or existing tablespace cannot be changed to a general tablespace.
There is no conflict between general tablespace names and file-per-table tablespace names. The “/” character, which is present in file-per-table tablespace names, is not permitted in general tablespace names.
mysqldump and mysqlpump
do not dump InnoDB
CREATE TABLESPACE
statements.
This example demonstrates creating a general tablespace and adding three uncompressed tables of different row formats.
mysql>CREATE TABLESPACE `ts1` ADD DATAFILE 'ts1.ibd' ENGINE=INNODB;
mysql>CREATE TABLE t1 (c1 INT PRIMARY KEY) TABLESPACE ts1 ROW_FORMAT=REDUNDANT;
mysql>CREATE TABLE t2 (c1 INT PRIMARY KEY) TABLESPACE ts1 ROW_FORMAT=COMPACT;
mysql>CREATE TABLE t3 (c1 INT PRIMARY KEY) TABLESPACE ts1 ROW_FORMAT=DYNAMIC;
This example demonstrates creating a general tablespace and adding
a compressed table. The example assumes a default
innodb_page_size
value of 16K.
The FILE_BLOCK_SIZE
of 8192 requires that the
compressed table have a KEY_BLOCK_SIZE
of 8.
mysql>CREATE TABLESPACE `ts2` ADD DATAFILE 'ts2.ibd' FILE_BLOCK_SIZE = 8192 Engine=InnoDB;
mysql>CREATE TABLE t4 (c1 INT PRIMARY KEY) TABLESPACE ts2 ROW_FORMAT=COMPRESSED KEY_BLOCK_SIZE=8;
This example demonstrates creating a general tablespace without
specifying the ADD DATAFILE
clause, which is
optional as of MySQL 8.0.14.
mysql> CREATE TABLESPACE `ts3` ENGINE=INNODB;
This example demonstrates creating an undo tablespace.
mysql> CREATE UNDO TABLESPACE undo_003
ADD DATAFILE 'undo_003
.ibu';
Suppose that you wish to create an NDB Cluster Disk Data
tablespace named myts
using a datafile named
mydata-1.dat
. An NDB
tablespace always requires the use of a log file group consisting
of one or more undo log files. For this example, we first create a
log file group named mylg
that contains one
undo long file named myundo-1.dat
, using the
CREATE LOGFILE GROUP
statement
shown here:
mysql>CREATE LOGFILE GROUP myg1
->ADD UNDOFILE 'myundo-1.dat'
->ENGINE=NDB;
Query OK, 0 rows affected (3.29 sec)
Now you can create the tablespace previously described using the following statement:
mysql>CREATE TABLESPACE myts
->ADD DATAFILE 'mydata-1.dat'
->USE LOGFILE GROUP mylg
->ENGINE=NDB;
Query OK, 0 rows affected (2.98 sec)
You can now create a Disk Data table using a
CREATE TABLE
statement with the
TABLESPACE
and STORAGE DISK
options, similar to what is shown here:
mysql>CREATE TABLE mytable (
->id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,
->lname VARCHAR(50) NOT NULL,
->fname VARCHAR(50) NOT NULL,
->dob DATE NOT NULL,
->joined DATE NOT NULL,
->INDEX(last_name, first_name)
->)
->TABLESPACE myts STORAGE DISK
->ENGINE=NDB;
Query OK, 0 rows affected (1.41 sec)
It is important to note that only the dob
and
joined
columns from mytable
are actually stored on disk, due to the fact that the
id
, lname
, and
fname
columns are all indexed.
As mentioned previously, when CREATE TABLESPACE
is used with ENGINE [=] NDB
, a tablespace and
associated data file are created on each NDB Cluster data node.
You can verify that the data files were created and obtain
information about them by querying the
INFORMATION_SCHEMA.FILES
table, as
shown here:
mysql>SELECT FILE_NAME, FILE_TYPE, LOGFILE_GROUP_NAME, STATUS, EXTRA
->FROM INFORMATION_SCHEMA.FILES
->WHERE TABLESPACE_NAME = 'myts';
+--------------+------------+--------------------+--------+----------------+ | file_name | file_type | logfile_group_name | status | extra | +--------------+------------+--------------------+--------+----------------+ | mydata-1.dat | DATAFILE | mylg | NORMAL | CLUSTER_NODE=5 | | mydata-1.dat | DATAFILE | mylg | NORMAL | CLUSTER_NODE=6 | | NULL | TABLESPACE | mylg | NORMAL | NULL | +--------------+------------+--------------------+--------+----------------+ 3 rows in set (0.01 sec)
For additional information and examples, see Section 22.5.13.1, “NDB Cluster Disk Data Objects”.
CREATE [DEFINER = {user
| CURRENT_USER }] TRIGGERtrigger_name
trigger_time
trigger_event
ONtbl_name
FOR EACH ROW [trigger_order
]trigger_body
trigger_time
: { BEFORE | AFTER }trigger_event
: { INSERT | UPDATE | DELETE }trigger_order
: { FOLLOWS | PRECEDES }other_trigger_name
This statement creates a new trigger. A trigger is a named
database object that is associated with a table, and that
activates when a particular event occurs for the table. The
trigger becomes associated with the table named
tbl_name
, which must refer to a
permanent table. You cannot associate a trigger with a
TEMPORARY
table or a view.
Trigger names exist in the schema namespace, meaning that all triggers must have unique names within a schema. Triggers in different schemas can have the same name.
This section describes CREATE
TRIGGER
syntax. For additional discussion, see
Section 24.3.1, “Trigger Syntax and Examples”.
CREATE TRIGGER
requires the
TRIGGER
privilege for the table
associated with the trigger. The statement might also require the
SET_USER_ID
or
SUPER
privilege, depending on the
DEFINER
value, as described later in this
section. If binary logging is enabled, CREATE
TRIGGER
might require the
SUPER
privilege, as described in
Section 24.7, “Binary Logging of Stored Programs”.
The DEFINER
clause determines the security
context to be used when checking access privileges at trigger
activation time, as described later in this section.
trigger_time
is the trigger action
time. It can be BEFORE
or
AFTER
to indicate that the trigger activates
before or after each row to be modified.
Basic column value checks occur prior to trigger activation, so
you cannot use BEFORE
triggers to convert
values inappropriate for the column type to valid values.
trigger_event
indicates the kind of
operation that activates the trigger. These
trigger_event
values are permitted:
INSERT
: The trigger activates
whenever a new row is inserted into the table; for example,
through INSERT
,
LOAD DATA
, and
REPLACE
statements.
UPDATE
: The trigger activates
whenever a row is modified; for example, through
UPDATE
statements.
DELETE
: The trigger activates
whenever a row is deleted from the table; for example, through
DELETE
and
REPLACE
statements.
DROP TABLE
and
TRUNCATE TABLE
statements on
the table do not activate this trigger,
because they do not use DELETE
.
Dropping a partition does not activate
DELETE
triggers, either.
The trigger_event
does not represent a
literal type of SQL statement that activates the trigger so much
as it represents a type of table operation. For example, an
INSERT
trigger activates not only
for INSERT
statements but also
LOAD DATA
statements because both
statements insert rows into a table.
A potentially confusing example of this is the INSERT
INTO ... ON DUPLICATE KEY UPDATE ...
syntax: a
BEFORE INSERT
trigger activates for every row,
followed by either an AFTER INSERT
trigger or
both the BEFORE UPDATE
and AFTER
UPDATE
triggers, depending on whether there was a
duplicate key for the row.
Cascaded foreign key actions do not activate triggers.
It is possible to define multiple triggers for a given table that
have the same trigger event and action time. For example, you can
have two BEFORE UPDATE
triggers for a table. By
default, triggers that have the same trigger event and action time
activate in the order they were created. To affect trigger order,
specify a trigger_order
clause that
indicates FOLLOWS
or
PRECEDES
and the name of an existing trigger
that also has the same trigger event and action time. With
FOLLOWS
, the new trigger activates after the
existing trigger. With PRECEDES
, the new
trigger activates before the existing trigger.
trigger_body
is the statement to
execute when the trigger activates. To execute multiple
statements, use the
BEGIN ... END
compound statement construct. This also enables you to use the
same statements that are permitted within stored routines. See
Section 13.6.1, “BEGIN ... END Compound-Statement Syntax”. Some statements are not permitted in
triggers; see Section C.1, “Restrictions on Stored Programs”.
Within the trigger body, you can refer to columns in the subject
table (the table associated with the trigger) by using the aliases
OLD
and NEW
.
OLD.
refers
to a column of an existing row before it is updated or deleted.
col_name
NEW.
refers
to the column of a new row to be inserted or an existing row after
it is updated.
col_name
Triggers cannot use
NEW.
or use
col_name
OLD.
to
refer to generated columns. For information about generated
columns, see Section 13.1.20.9, “CREATE TABLE and Generated Columns”.
col_name
MySQL stores the sql_mode
system
variable setting in effect when a trigger is created, and always
executes the trigger body with this setting in force,
regardless of the current server SQL mode when the
trigger begins executing.
The DEFINER
clause specifies the MySQL account
to be used when checking access privileges at trigger activation
time. If a user
value is given, it
should be a MySQL account specified as
'
,
user_name
'@'host_name
'CURRENT_USER
, or
CURRENT_USER()
. The default
DEFINER
value is the user who executes the
CREATE TRIGGER
statement. This is
the same as specifying DEFINER = CURRENT_USER
explicitly.
If you specify the DEFINER
clause, these rules
determine the valid DEFINER
user values:
If you do not have the
SET_USER_ID
or
SUPER
privilege, the only
permitted user
value is your own
account, either specified literally or by using
CURRENT_USER
. You cannot set
the definer to some other account.
If you have the SET_USER_ID
or
SUPER
privilege, you can
specify any syntactically valid account name. If the account
does not exist, a warning is generated.
Although it is possible to create a trigger with a nonexistent
DEFINER
account, it is not a good idea for
such triggers to be activated until the account actually does
exist. Otherwise, the behavior with respect to privilege
checking is undefined.
MySQL takes the DEFINER
user into account when
checking trigger privileges as follows:
At CREATE TRIGGER
time, the
user who issues the statement must have the
TRIGGER
privilege.
At trigger activation time, privileges are checked against the
DEFINER
user. This user must have these
privileges:
The TRIGGER
privilege for
the subject table.
The SELECT
privilege for
the subject table if references to table columns occur
using
OLD.
or
col_name
NEW.
in the trigger body.
col_name
The UPDATE
privilege for
the subject table if table columns are targets of
SET NEW.
assignments in
the trigger body.
col_name
=
value
Whatever other privileges normally are required for the statements executed by the trigger.
For more information about trigger security, see Section 24.6, “Access Control for Stored Programs and Views”.
Within a trigger body, the
CURRENT_USER()
function returns the
account used to check privileges at trigger activation time. This
is the DEFINER
user, not the user whose actions
caused the trigger to be activated. For information about user
auditing within triggers, see
Section 6.3.13, “SQL-Based MySQL Account Activity Auditing”.
If you use LOCK TABLES
to lock a
table that has triggers, the tables used within the trigger are
also locked, as described in
LOCK TABLES and Triggers.
For additional discussion of trigger use, see Section 24.3.1, “Trigger Syntax and Examples”.
CREATE [OR REPLACE] [ALGORITHM = {UNDEFINED | MERGE | TEMPTABLE}] [DEFINER = {user
| CURRENT_USER }] [SQL SECURITY { DEFINER | INVOKER }] VIEWview_name
[(column_list
)] ASselect_statement
[WITH [CASCADED | LOCAL] CHECK OPTION]
The CREATE VIEW
statement creates a
new view, or replaces an existing view if the OR
REPLACE
clause is given. If the view does not exist,
CREATE OR REPLACE
VIEW
is the same as CREATE
VIEW
. If the view does exist,
CREATE OR REPLACE
VIEW
replaces it.
For information about restrictions on view use, see Section C.5, “Restrictions on Views”.
The select_statement
is a
SELECT
statement that provides the
definition of the view. (Selecting from the view selects, in
effect, using the SELECT
statement.) The select_statement
can
select from base tables or other views.
The view definition is “frozen” at creation time and
is not affected by subsequent changes to the definitions of the
underlying tables. For example, if a view is defined as
SELECT *
on a table, new columns added to the
table later do not become part of the view, and columns dropped
from the table will result in an error when selecting from the
view.
The ALGORITHM
clause affects how MySQL
processes the view. The DEFINER
and
SQL SECURITY
clauses specify the security
context to be used when checking access privileges at view
invocation time. The WITH CHECK OPTION
clause
can be given to constrain inserts or updates to rows in tables
referenced by the view. These clauses are described later in this
section.
The CREATE VIEW
statement requires
the CREATE VIEW
privilege for the
view, and some privilege for each column selected by the
SELECT
statement. For columns used
elsewhere in the SELECT
statement,
you must have the SELECT
privilege.
If the OR REPLACE
clause is present, you must
also have the DROP
privilege for
the view. CREATE VIEW
might also
require the SET_USER_ID
or
SUPER
privilege, depending on the
DEFINER
value, as described later in this
section.
When a view is referenced, privilege checking occurs as described later in this section.
A view belongs to a database. By default, a new view is created in
the default database. To create the view explicitly in a given
database, use db_name.view_name
syntax
to qualify the view name with the database name:
CREATE VIEW test.v AS SELECT * FROM t;
Unqualified table or view names in the
SELECT
statement are also
interpreted with respect to the default database. A view can refer
to tables or views in other databases by qualifying the table or
view name with the appropriate database name.
Within a database, base tables and views share the same namespace, so a base table and a view cannot have the same name.
Columns retrieved by the SELECT
statement can be simple references to table columns, or
expressions that use functions, constant values, operators, and so
forth.
A view must have unique column names with no duplicates, just like
a base table. By default, the names of the columns retrieved by
the SELECT
statement are used for
the view column names. To define explicit names for the view
columns, specify the optional
column_list
clause as a list of
comma-separated identifiers. The number of names in
column_list
must be the same as the
number of columns retrieved by the
SELECT
statement.
A view can be created from many kinds of
SELECT
statements. It can refer to
base tables or other views. It can use joins,
UNION
, and subqueries. The
SELECT
need not even refer to any
tables:
CREATE VIEW v_today (today) AS SELECT CURRENT_DATE;
The following example defines a view that selects two columns from another table as well as an expression calculated from those columns:
mysql>CREATE TABLE t (qty INT, price INT);
mysql>INSERT INTO t VALUES(3, 50);
mysql>CREATE VIEW v AS SELECT qty, price, qty*price AS value FROM t;
mysql>SELECT * FROM v;
+------+-------+-------+ | qty | price | value | +------+-------+-------+ | 3 | 50 | 150 | +------+-------+-------+
A view definition is subject to the following restrictions:
The SELECT
statement cannot
refer to system variables or user-defined variables.
Within a stored program, the
SELECT
statement cannot refer
to program parameters or local variables.
The SELECT
statement cannot
refer to prepared statement parameters.
Any table or view referred to in the definition must exist.
If, after the view has been created, a table or view that the
definition refers to is dropped, use of the view results in an
error. To check a view definition for problems of this kind,
use the CHECK TABLE
statement.
The definition cannot refer to a TEMPORARY
table, and you cannot create a TEMPORARY
view.
You cannot associate a trigger with a view.
Aliases for column names in the
SELECT
statement are checked
against the maximum column length of 64 characters (not the
maximum alias length of 256 characters).
ORDER BY
is permitted in a view definition, but
it is ignored if you select from a view using a statement that has
its own ORDER BY
.
For other options or clauses in the definition, they are added to
the options or clauses of the statement that references the view,
but the effect is undefined. For example, if a view definition
includes a LIMIT
clause, and you select from
the view using a statement that has its own
LIMIT
clause, it is undefined which limit
applies. This same principle applies to options such as
ALL
, DISTINCT
, or
SQL_SMALL_RESULT
that follow the
SELECT
keyword, and to clauses such
as INTO
, FOR UPDATE
,
FOR SHARE
, LOCK IN SHARE
MODE
, and PROCEDURE
.
The results obtained from a view may be affected if you change the query processing environment by changing system variables:
mysql>CREATE VIEW v (mycol) AS SELECT 'abc';
Query OK, 0 rows affected (0.01 sec) mysql>SET sql_mode = '';
Query OK, 0 rows affected (0.00 sec) mysql>SELECT "mycol" FROM v;
+-------+ | mycol | +-------+ | mycol | +-------+ 1 row in set (0.01 sec) mysql>SET sql_mode = 'ANSI_QUOTES';
Query OK, 0 rows affected (0.00 sec) mysql>SELECT "mycol" FROM v;
+-------+ | mycol | +-------+ | abc | +-------+ 1 row in set (0.00 sec)
The DEFINER
and SQL SECURITY
clauses determine which MySQL account to use when checking access
privileges for the view when a statement is executed that
references the view. The valid SQL SECURITY
characteristic values are DEFINER
(the default)
and INVOKER
. These indicate that the required
privileges must be held by the user who defined or invoked the
view, respectively.
If a user
value is given for the
DEFINER
clause, it should be a MySQL account
specified as
'
,
user_name
'@'host_name
'CURRENT_USER
, or
CURRENT_USER()
. The default
DEFINER
value is the user who executes the
CREATE VIEW
statement. This is the
same as specifying DEFINER = CURRENT_USER
explicitly.
If the DEFINER
clause is present, these rules
determine the valid DEFINER
user values:
If you do not have the
SET_USER_ID
or
SUPER
privilege, the only valid
user
value is your own account,
either specified literally or by using
CURRENT_USER
. You cannot set
the definer to some other account.
If you have the SET_USER_ID
or
SUPER
privilege, you can
specify any syntactically valid account name. If the account
does not exist, a warning is generated.
Although it is possible to create a view with a nonexistent
DEFINER
account, an error occurs when the
view is referenced if the SQL SECURITY
value is DEFINER
but the definer account
does not exist.
For more information about view security, see Section 24.6, “Access Control for Stored Programs and Views”.
Within a view definition,
CURRENT_USER
returns the view's
DEFINER
value by default. For views defined
with the SQL SECURITY INVOKER
characteristic,
CURRENT_USER
returns the account
for the view's invoker. For information about user auditing within
views, see Section 6.3.13, “SQL-Based MySQL Account Activity Auditing”.
Within a stored routine that is defined with the SQL
SECURITY DEFINER
characteristic,
CURRENT_USER
returns the routine's
DEFINER
value. This also affects a view defined
within such a routine, if the view definition contains a
DEFINER
value of
CURRENT_USER
.
MySQL checks view privileges like this:
At view definition time, the view creator must have the
privileges needed to use the top-level objects accessed by the
view. For example, if the view definition refers to table
columns, the creator must have some privilege for each column
in the select list of the definition, and the
SELECT
privilege for each
column used elsewhere in the definition. If the definition
refers to a stored function, only the privileges needed to
invoke the function can be checked. The privileges required at
function invocation time can be checked only as it executes:
For different invocations, different execution paths within
the function might be taken.
The user who references a view must have appropriate
privileges to access it (SELECT
to select from it, INSERT
to
insert into it, and so forth.)
When a view has been referenced, privileges for objects
accessed by the view are checked against the privileges held
by the view DEFINER
account or invoker,
depending on whether the SQL SECURITY
characteristic is DEFINER
or
INVOKER
, respectively.
If reference to a view causes execution of a stored function,
privilege checking for statements executed within the function
depend on whether the function SQL SECURITY
characteristic is DEFINER
or
INVOKER
. If the security characteristic is
DEFINER
, the function runs with the
privileges of the DEFINER
account. If the
characteristic is INVOKER
, the function
runs with the privileges determined by the view's SQL
SECURITY
characteristic.
Example: A view might depend on a stored function, and that
function might invoke other stored routines. For example, the
following view invokes a stored function f()
:
CREATE VIEW v AS SELECT * FROM t WHERE t.id = f(t.name);
Suppose that f()
contains a statement such as
this:
IF name IS NULL then CALL p1(); ELSE CALL p2(); END IF;
The privileges required for executing statements within
f()
need to be checked when
f()
executes. This might mean that privileges
are needed for p1()
or p2()
,
depending on the execution path within f()
.
Those privileges must be checked at runtime, and the user who must
possess the privileges is determined by the SQL
SECURITY
values of the view v
and the
function f()
.
The DEFINER
and SQL SECURITY
clauses for views are extensions to standard SQL. In standard SQL,
views are handled using the rules for SQL SECURITY
DEFINER
. The standard says that the definer of the view,
which is the same as the owner of the view's schema, gets
applicable privileges on the view (for example,
SELECT
) and may grant them. MySQL
has no concept of a schema “owner”, so MySQL adds a
clause to identify the definer. The DEFINER
clause is an extension where the intent is to have what the
standard has; that is, a permanent record of who defined the view.
This is why the default DEFINER
value is the
account of the view creator.
The optional ALGORITHM
clause is a MySQL
extension to standard SQL. It affects how MySQL processes the
view. ALGORITHM
takes three values:
MERGE
, TEMPTABLE
, or
UNDEFINED
. For more information, see
Section 24.5.2, “View Processing Algorithms”, as well as
Section 8.2.2.4, “Optimizing Derived Tables, View References, and Common Table Expressions
with Merging or Materialization”.
Some views are updatable. That is, you can use them in statements
such as UPDATE
,
DELETE
, or
INSERT
to update the contents of
the underlying table. For a view to be updatable, there must be a
one-to-one relationship between the rows in the view and the rows
in the underlying table. There are also certain other constructs
that make a view nonupdatable.
A generated column in a view is considered updatable because it is
possible to assign to it. However, if such a column is updated
explicitly, the only permitted value is
DEFAULT
. For information about generated
columns, see Section 13.1.20.9, “CREATE TABLE and Generated Columns”.
The WITH CHECK OPTION
clause can be given for
an updatable view to prevent inserts or updates to rows except
those for which the WHERE
clause in the
select_statement
is true.
In a WITH CHECK OPTION
clause for an updatable
view, the LOCAL
and CASCADED
keywords determine the scope of check testing when the view is
defined in terms of another view. The LOCAL
keyword restricts the CHECK OPTION
only to the
view being defined. CASCADED
causes the checks
for underlying views to be evaluated as well. When neither keyword
is given, the default is CASCADED
.
For more information about updatable views and the WITH
CHECK OPTION
clause, see
Section 24.5.3, “Updatable and Insertable Views”, and
Section 24.5.4, “The View WITH CHECK OPTION Clause”.
DROP {DATABASE | SCHEMA} [IF EXISTS] db_name
DROP DATABASE
drops all tables in
the database and deletes the database. Be
very careful with this statement! To use
DROP DATABASE
, you need the
DROP
privilege on the database.
DROP
SCHEMA
is a synonym for DROP
DATABASE
.
When a database is dropped, privileges granted specifically for the database are not automatically dropped. They must be dropped manually. See Section 13.7.1.6, “GRANT Syntax”.
IF EXISTS
is used to prevent an error from
occurring if the database does not exist.
If the default database is dropped, the default database is unset
(the DATABASE()
function returns
NULL
).
If you use DROP DATABASE
on a
symbolically linked database, both the link and the original
database are deleted.
DROP DATABASE
returns the number of
tables that were removed.
The DROP DATABASE
statement removes
from the given database directory those files and directories that
MySQL itself may create during normal operation. This includes all
files with the extensions shown in the following list:
.BAK
.DAT
.HSH
.MRG
.MYD
.MYI
.cfg
.db
.ibd
.ndb
If other files or directories remain in the database directory
after MySQL removes those just listed, the database directory
cannot be removed. In this case, you must remove any remaining
files or directories manually and issue the
DROP DATABASE
statement again.
Dropping a database does not remove any
TEMPORARY
tables that were created in that
database. TEMPORARY
tables are automatically
removed when the session that created them ends. See
Section 13.1.20.3, “CREATE TEMPORARY TABLE Syntax”.
You can also drop databases with mysqladmin. See Section 4.5.2, “mysqladmin — Client for Administering a MySQL Server”.
DROP EVENT [IF EXISTS] event_name
This statement drops the event named
event_name
. The event immediately
ceases being active, and is deleted completely from the server.
If the event does not exist, the error ERROR 1517
(HY000): Unknown event
'event_name
' results. You
can override this and cause the statement to generate a warning
for nonexistent events instead using IF EXISTS
.
This statement requires the EVENT
privilege for the schema to which the event to be dropped belongs.
The DROP FUNCTION
statement is used
to drop stored functions and user-defined functions (UDFs):
For information about dropping stored functions, see Section 13.1.29, “DROP PROCEDURE and DROP FUNCTION Syntax”.
For information about dropping user-defined functions, see Section 13.7.4.2, “DROP FUNCTION Syntax”.
DROP INDEXindex_name
ONtbl_name
[algorithm_option
|lock_option
] ...algorithm_option
: ALGORITHM [=] {DEFAULT|INPLACE|COPY}lock_option
: LOCK [=] {DEFAULT|NONE|SHARED|EXCLUSIVE}
DROP INDEX
drops the index named
index_name
from the table
tbl_name
. This statement is mapped to
an ALTER TABLE
statement to drop
the index. See Section 13.1.9, “ALTER TABLE Syntax”.
To drop a primary key, the index name is always
PRIMARY
, which must be specified as a quoted
identifier because PRIMARY
is a reserved word:
DROP INDEX `PRIMARY` ON t;
Indexes on variable-width columns of
NDB
tables are dropped online; that
is, without any table copying. The table is not locked against
access from other NDB Cluster API nodes, although it is locked
against other operations on the same API node
for the duration of the operation. This is done automatically by
the server whenever it determines that it is possible to do so;
you do not have to use any special SQL syntax or server options to
cause it to happen.
ALGORITHM
and LOCK
clauses
may be given to influence the table copying method and level of
concurrency for reading and writing the table while its indexes
are being modified. They have the same meaning as for the
ALTER TABLE
statement. For more
information, see Section 13.1.9, “ALTER TABLE Syntax”
MySQL NDB Cluster supports online operations using the same
ALGORITHM=INPLACE
syntax supported in the
standard MySQL Server. See
Section 22.5.14, “Online Operations with ALTER TABLE in NDB Cluster”, for more
information.
DROP LOGFILE GROUPlogfile_group
ENGINE [=]engine_name
This statement drops the log file group named
logfile_group
. The log file group must
already exist or an error results. (For information on creating
log file groups, see Section 13.1.16, “CREATE LOGFILE GROUP Syntax”.)
Before dropping a log file group, you must drop all tablespaces
that use that log file group for UNDO
logging.
The required ENGINE
clause provides the name of
the storage engine used by the log file group to be dropped.
Currently, the only permitted values for
engine_name
are
NDB
and
NDBCLUSTER
.
DROP LOGFILE GROUP
is useful only
with Disk Data storage for NDB Cluster. See
Section 22.5.13, “NDB Cluster Disk Data Tables”.
DROP {PROCEDURE | FUNCTION} [IF EXISTS] sp_name
This statement is used to drop a stored procedure or function.
That is, the specified routine is removed from the server. You
must have the ALTER ROUTINE
privilege for the routine. (If the
automatic_sp_privileges
system variable is
enabled, that privilege and EXECUTE
are granted automatically to the routine creator when the routine
is created and dropped from the creator when the routine is
dropped. See Section 24.2.2, “Stored Routines and MySQL Privileges”.)
The IF EXISTS
clause is a MySQL extension. It
prevents an error from occurring if the procedure or function does
not exist. A warning is produced that can be viewed with
SHOW WARNINGS
.
DROP FUNCTION
is also used to drop
user-defined functions (see Section 13.7.4.2, “DROP FUNCTION Syntax”).
DROP SERVER [ IF EXISTS ] server_name
Drops the server definition for the server named
. The
corresponding row in the server_name
mysql.servers
table is
deleted. This statement requires the
SUPER
privilege.
Dropping a server for a table does not affect any
FEDERATED
tables that used this connection
information when they were created. See
Section 13.1.18, “CREATE SERVER Syntax”.
DROP SERVER
causes an implicit commit. See
Section 13.3.3, “Statements That Cause an Implicit Commit”.
DROP SERVER
is not written to the binary log,
regardless of the logging format that is in use.
DROP SPATIAL REFERENCE SYSTEM [IF EXISTS]srid
srid
:32-bit unsigned integer
This statement removes a
spatial reference
system (SRS) definition from the data dictionary. It
requires the SUPER
privilege.
Example:
DROP SPATIAL REFERENCE SYSTEM 4120;
If no SRS definition with the SRID value exists, an error occurs
unless IF EXISTS
is specified. In that case, a
warning occurs rather than an error.
If the SRID value is used by some column in an existing table, an error occurs. For example:
mysql> DROP SPATIAL REFERENCE SYSTEM 4326;
ERROR 3716 (SR005): Can't modify SRID 4326. There is at
least one column depending on it.
To identify which column or columns use the SRID, use this query:
SELECT * FROM INFORMATION_SCHEMA.ST_GEOMETRY_COLUMNS WHERE SRS_ID=4326;
SRID values must be in the range of 32-bit unsigned integers, with these restrictions:
SRID 0 is a valid SRID but cannot be used with
DROP SPATIAL REFERENCE SYSTEM
.
If the value is in a reserved SRID range, a warning occurs. Reserved ranges are [0, 32767] (reserved by EPSG), [60,000,000, 69,999,999] (reserved by EPSG), and [2,000,000,000, 2,147,483,647] (reserved by MySQL). EPSG stands for the European Petroleum Survey Group.
Users should not drop SRSs with SRIDs in the reserved ranges. If system-installed SRSs are dropped, the SRS definitions may be recreated for MySQL upgrades.
DROP [TEMPORARY] TABLE [IF EXISTS]tbl_name
[,tbl_name
] ... [RESTRICT | CASCADE]
DROP TABLE
removes one or more
tables. You must have the DROP
privilege for each table.
Be careful with this statement! It removes the table definition and all table data. For a partitioned table, it permanently removes the table definition, all its partitions, and all data stored in those partitions. It also removes partition definitions associated with the dropped table.
Dropping a table also drops any triggers for the table.
DROP TABLE
causes an implicit
commit, except when used with the TEMPORARY
keyword. See Section 13.3.3, “Statements That Cause an Implicit Commit”.
When a table is dropped, privileges granted specifically for the table are not automatically dropped. They must be dropped manually. See Section 13.7.1.6, “GRANT Syntax”.
If any tables named in the argument list do not exist, the statement fails with an error indicating by name which nonexisting tables it was unable to drop, and no changes are made.
Use IF EXISTS
to prevent an error from
occurring for tables that do not exist. Instead of an error, a
NOTE
is generated for each nonexistent table;
these notes can be displayed with SHOW
WARNINGS
. See Section 13.7.6.40, “SHOW WARNINGS Syntax”.
IF EXISTS
can also be useful for dropping
tables in unusual circumstances under which there is an entry in
the data dictionary but no table managed by the storage engine.
(For example, if an abnormal server exit occurs after removal of
the table from the storage engine but before removal of the data
dictionary entry.)
The TEMPORARY
keyword has the following
effects:
The statement drops only TEMPORARY
tables.
The statement does not cause an implicit commit.
No access rights are checked. A TEMPORARY
table is visible only with the session that created it, so no
check is necessary.
Using TEMPORARY
is a good way to ensure that
you do not accidentally drop a non-TEMPORARY
table.
The RESTRICT
and CASCADE
keywords do nothing. They are permitted to make porting easier
from other database systems.
DROP TABLE
is not supported with
all innodb_force_recovery
settings. See Section 15.20.2, “Forcing InnoDB Recovery”.
DROP [UNDO] TABLESPACEtablespace_name
[ENGINE [=]engine_name
]
This statement drops a tablespace that was previously created
using CREATE TABLESPACE
. It is
supported by the NDB
and
InnoDB
storage engines.
The UNDO
keyword, introduced in MySQL 8.0.14,
must be specified to drop an undo tablespace. Only undo
tablespaces created using
CREATE UNDO
TABLESPACE
syntax can be dropped. An undo tablespace
must be in an empty
state before it can be
dropped. For more information, see
Section 15.6.3.4, “Undo Tablespaces”.
ENGINE
sets the storage engine that uses the
tablespace, where engine_name
is the
name of the storage engine. Currently, the values
InnoDB
and NDB
are
supported. If not set, the value of
default_storage_engine
is used.
If it is not the same as the storage engine used to create the
tablespace, the DROP TABLESPACE
statement
fails.
is a
case-sensitive identifier in MySQL.
tablespace_name
For an InnoDB
general tablespace, all tables
must be dropped from the tablespace prior to a DROP
TABLESPACE
operation. If the tablespace is not empty,
DROP TABLESPACE
returns an error.
An NDB
tablespace to be dropped must not
contain any data files; in other words, before you can drop an
NDB
tablespace, you must first drop each of its
data files using
ALTER TABLESPACE
... DROP DATAFILE
.
A general InnoDB
tablespace is not deleted
automatically when the last table in the tablespace is
dropped. The tablespace must be dropped explicitly using
DROP TABLESPACE
.
tablespace_name
A DROP DATABASE
operation can
drop tables that belong to a general tablespace but it cannot
drop the tablespace, even if the operation drops all tables
that belong to the tablespace. The tablespace must be dropped
explicitly using DROP TABLESPACE
.
tablespace_name
Similar to the system tablespace, truncating or dropping
tables stored in a general tablespace creates free space
internally in the general tablespace
.ibd data file which can
only be used for new InnoDB
data. Space is
not released back to the operating system as it is for
file-per-table tablespaces.
This example demonstrates how to drop an InnoDB
general tablespace. The general tablespace ts1
is created with a single table. Before dropping the tablespace,
the table must be dropped.
mysql>CREATE TABLESPACE `ts1` ADD DATAFILE 'ts1.ibd' Engine=InnoDB;
mysql>CREATE TABLE t1 (c1 INT PRIMARY KEY) TABLESPACE ts10 Engine=InnoDB;
mysql>DROP TABLE t1;
mysql>DROP TABLESPACE ts1;
This example demonstrates dropping an undo tablespace. An undo
tablespace must be in an empty
state before it
can be dropped. For more information, see
Section 15.6.3.4, “Undo Tablespaces”.
mysql> DROP UNDO TABLESPACE undo_003
;
This example shows how to drop an NDB
tablespace myts
having a data file named
mydata-1.dat
after first creating the
tablespace, and assumes the existence of a log file group named
mylg
(see
Section 13.1.16, “CREATE LOGFILE GROUP Syntax”).
mysql>CREATE TABLESPACE myts
->ADD DATAFILE 'mydata-1.dat'
->USE LOGFILE GROUP mylg
->ENGINE=NDB;
You must remove all data files from the tablespace using
ALTER TABLESPACE
, as shown here,
before it can be dropped:
mysql>ALTER TABLESPACE myts
->DROP DATAFILE 'mydata-1.dat'
->ENGINE=NDB;
mysql>DROP TABLESPACE myts;
DROP TRIGGER [IF EXISTS] [schema_name
.]trigger_name
This statement drops a trigger. The schema (database) name is
optional. If the schema is omitted, the trigger is dropped from
the default schema. DROP TRIGGER
requires the TRIGGER
privilege for
the table associated with the trigger.
Use IF EXISTS
to prevent an error from
occurring for a trigger that does not exist. A
NOTE
is generated for a nonexistent trigger
when using IF EXISTS
. See
Section 13.7.6.40, “SHOW WARNINGS Syntax”.
Triggers for a table are also dropped if you drop the table.
DROP VIEW [IF EXISTS]view_name
[,view_name
] ... [RESTRICT | CASCADE]
DROP VIEW
removes one or more
views. You must have the DROP
privilege for each view.
If any views named in the argument list do not exist, the statement fails with an error indicating by name which nonexisting views it was unable to drop, and no changes are made.
In MySQL 5.7 and earlier,
DROP VIEW
returns an error if any
views named in the argument list do not exist, but also drops
all views in the list that do exist. Due to the change in
behavior in MySQL 8.0, a partially completed
DROP VIEW
operation on a MySQL
5.7 master fails when replicated on a MySQL
8.0 slave. To avoid this failure scenario, use
IF EXISTS
syntax in DROP
VIEW
statements to prevent an error from occurring for
views that do not exist. For more information, see
Section 13.1.1, “Atomic Data Definition Statement Support”.
The IF EXISTS
clause prevents an error from
occurring for views that don't exist. When this clause is given, a
NOTE
is generated for each nonexistent view.
See Section 13.7.6.40, “SHOW WARNINGS Syntax”.
RESTRICT
and CASCADE
, if
given, are parsed and ignored.
RENAME TABLEtbl_name
TOnew_tbl_name
[,tbl_name2
TOnew_tbl_name2
] ...
RENAME TABLE
renames one or more
tables. You must have ALTER
and
DROP
privileges for the original
table, and CREATE
and
INSERT
privileges for the new
table.
For example, to rename a table named old_table
to to new_table
, use this statement:
RENAME TABLE old_table TO new_table;
That statement is equivalent to the following
ALTER TABLE
statement:
ALTER TABLE old_table RENAME new_table;
RENAME TABLE
, unlike ALTER
TABLE
, can rename multiple tables within a single
statement:
RENAME TABLE old_table1 TO new_table1, old_table2 TO new_table2, old_table3 TO new_table3;
Renaming operations are performed left to right. Thus, to swap two
table names, do this (assuming that a table with the intermediary
name tmp_table
does not already exist):
RENAME TABLE old_table TO tmp_table, new_table TO old_table, tmp_table TO new_table;
Metadata locks on tables are acquired in name order, which in some cases can make a difference in operation outcome when multiple transactions execute concurrently. See Section 8.11.4, “Metadata Locking”.
As of MySQL 8.0.13, you can rename tables locked with a
LOCK TABLES
statement, provided
that they are locked with a WRITE
lock or are
the product of renaming WRITE
-locked tables
from earlier steps in a multiple-table rename operation. For
example, this is permitted:
LOCK TABLE old_table1 WRITE; RENAME TABLE old_table1 TO new_table1, new_table1 TO new_table2;
This is not permitted:
LOCK TABLE old_table1 READ; RENAME TABLE old_table1 TO new_table1, new_table1 TO new_table2;
Prior to MySQL 8.0.13, to execute RENAME TABLE
,
there must be no tables locked with LOCK
TABLES
.
With the transaction table locking conditions satisfied, the rename operation is done atomically; no other session can access any of the tables while the rename is in progress.
If any errors occur during a RENAME TABLE
, the
statement fails and no changes are made.
You can use RENAME TABLE
to move a table from
one database to another:
RENAME TABLEcurrent_db.tbl_name
TOother_db.tbl_name;
Using this method to move all tables from one database to a different one in effect renames the database (an operation for which MySQL has no single statement), except that the original database continues to exist, albeit with no tables.
Like RENAME TABLE
, ALTER TABLE ...
RENAME
can also be used to move a table to a different
database. Regardless of the statement used, if the rename
operation would move the table to a database located on a
different file system, the success of the outcome is platform
specific and depends on the underlying operating system calls used
to move table files.
If a table has triggers, attempts to rename the table into a
different database fail with a Trigger in wrong
schema
(ER_TRG_IN_WRONG_SCHEMA
) error.
To rename TEMPORARY
tables, RENAME
TABLE
does not work. Use ALTER
TABLE
instead.
RENAME TABLE
works for views, except that views
cannot be renamed into a different database.
Any privileges granted specifically for a renamed table or view are not migrated to the new name. They must be changed manually.
RENAME TABLE
changes
internally generated foreign key constraint names and user-defined
foreign key constraint names that begin with the string
“tbl_name
TO
new_tbl_name
tbl_name
_ibfk_” to
reflect the new table name. InnoDB
interprets
foreign key constraint names that begin with the string
“tbl_name
_ibfk_” as
internally generated names.
Foreign key constraint names that point to the renamed table are automatically updated unless there is a conflict, in which case the statement fails with an error. A conflict occurs if the renamed constraint name already exists. In such cases, you must drop and re-create the foreign keys for them to function properly.
RENAME TABLE
changes
internally generated and user-defined tbl_name
TO
new_tbl_name
CHECK
constraint names that begin with the string
“tbl_name
_chk_” to reflect
the new table name. MySQL interprets CHECK
constraint names that begin with the string
“tbl_name
_chk_” as
internally generated names. Example:
mysql>SHOW CREATE TABLE t1\G
*************************** 1. row *************************** Table: t1 Create Table: CREATE TABLE `t1` ( `i1` int(11) DEFAULT NULL, `i2` int(11) DEFAULT NULL, CONSTRAINT `t1_chk_1` CHECK ((`i1` > 0)), CONSTRAINT `t1_chk_2` CHECK ((`i2` < 0)) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci 1 row in set (0.02 sec) mysql>RENAME TABLE t1 TO t3;
Query OK, 0 rows affected (0.03 sec) mysql>SHOW CREATE TABLE t3\G
*************************** 1. row *************************** Table: t3 Create Table: CREATE TABLE `t3` ( `i1` int(11) DEFAULT NULL, `i2` int(11) DEFAULT NULL, CONSTRAINT `t3_chk_1` CHECK ((`i1` > 0)), CONSTRAINT `t3_chk_2` CHECK ((`i2` < 0)) ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci 1 row in set (0.01 sec)
TRUNCATE [TABLE] tbl_name
TRUNCATE TABLE
empties a table
completely. It requires the DROP
privilege. Logically, TRUNCATE
TABLE
is similar to a
DELETE
statement that deletes all
rows, or a sequence of DROP TABLE
and CREATE TABLE
statements.
To achieve high performance, TRUNCATE
TABLE
bypasses the DML method of deleting data. Thus, it
does not cause ON DELETE
triggers to fire, it
cannot be performed for InnoDB
tables with
parent-child foreign key relationships, and it cannot be rolled
back like a DML operation. However, TRUNCATE
TABLE
operations on tables that use an atomic
DDL-supported storage engine are either fully committed or rolled
back if the server halts during their operation. For more
information, see Section 13.1.1, “Atomic Data Definition Statement Support”.
Although TRUNCATE TABLE
is similar
to DELETE
, it is classified as a
DDL statement rather than a DML statement. It differs from
DELETE
in the following ways:
Truncate operations drop and re-create the table, which is much faster than deleting rows one by one, particularly for large tables.
Truncate operations cause an implicit commit, and so cannot be rolled back. See Section 13.3.3, “Statements That Cause an Implicit Commit”.
Truncation operations cannot be performed if the session holds an active table lock.
TRUNCATE TABLE
fails for an
InnoDB
table or
NDB
table if there are any
FOREIGN KEY
constraints from other tables
that reference the table. Foreign key constraints between
columns of the same table are permitted.
Truncation operations do not return a meaningful value for the number of deleted rows. The usual result is “0 rows affected,” which should be interpreted as “no information.”
As long as the table definition is valid, the table can be
re-created as an empty table with
TRUNCATE TABLE
, even if the
data or index files have become corrupted.
Any AUTO_INCREMENT
value is reset to its
start value. This is true even for MyISAM
and InnoDB
, which normally do not reuse
sequence values.
When used with partitioned tables,
TRUNCATE TABLE
preserves the
partitioning; that is, the data and index files are dropped
and re-created, while the partition definitions are
unaffected.
The TRUNCATE TABLE
statement
does not invoke ON DELETE
triggers.
Truncating a corrupted InnoDB
table is
supported.
TRUNCATE TABLE
for a table closes
all handlers for the table that were opened with
HANDLER OPEN
.
TRUNCATE TABLE
is treated for
purposes of binary logging and replication as
DROP TABLE
followed by
CREATE TABLE
—that is, as DDL
rather than DML. This is due to the fact that, when using
InnoDB
and other transactional
storage engines where the transaction isolation level does not
permit statement-based logging (READ
COMMITTED
or READ
UNCOMMITTED
), the statement was not logged and
replicated when using STATEMENT
or
MIXED
logging mode. (Bug #36763) However, it is
still applied on replication slaves using
InnoDB
in the manner described
previously.
In MySQL 5.7 and earlier, on a system with a large buffer pool and
innodb_adaptive_hash_index
enabled, a TRUNCATE TABLE
operation could cause
a temporary drop in system performance due to an LRU scan that
occurred when removing the table's adaptive hash index entries
(Bug #68184). The remapping of TRUNCATE
TABLE
to DROP TABLE
and
CREATE TABLE
in MySQL 8.0 avoids
the problematic LRU scan.
TRUNCATE TABLE
can be used with
Performance Schema summary tables, but the effect is to reset the
summary columns to 0 or NULL
, not to remove
rows. See Section 26.12.16, “Performance Schema Summary Tables”.
CALLsp_name
([parameter
[,...]]) CALLsp_name
[()]
The CALL
statement invokes a stored
procedure that was defined previously with
CREATE PROCEDURE
.
Stored procedures that take no arguments can be invoked without
parentheses. That is, CALL p()
and
CALL p
are equivalent.
CALL
can pass back values to its
caller using parameters that are declared as
OUT
or INOUT
parameters.
When the procedure returns, a client program can also obtain the
number of rows affected for the final statement executed within
the routine: At the SQL level, call the
ROW_COUNT()
function; from the C
API, call the
mysql_affected_rows()
function.
For information about the effect of unhandled conditions on procedure parameters, see Section 13.6.7.8, “Condition Handling and OUT or INOUT Parameters”.
To get back a value from a procedure using an
OUT
or INOUT
parameter, pass
the parameter by means of a user variable, and then check the
value of the variable after the procedure returns. (If you are
calling the procedure from within another stored procedure or
function, you can also pass a routine parameter or local routine
variable as an IN
or INOUT
parameter.) For an INOUT
parameter, initialize
its value before passing it to the procedure. The following
procedure has an OUT
parameter that the
procedure sets to the current server version, and an
INOUT
value that the procedure increments by
one from its current value:
CREATE PROCEDURE p (OUT ver_param VARCHAR(25), INOUT incr_param INT) BEGIN # Set value of OUT parameter SELECT VERSION() INTO ver_param; # Increment value of INOUT parameter SET incr_param = incr_param + 1; END;
Before calling the procedure, initialize the variable to be passed
as the INOUT
parameter. After calling the
procedure, the values of the two variables will have been set or
modified:
mysql>SET @increment = 10;
mysql>CALL p(@version, @increment);
mysql>SELECT @version, @increment;
+--------------------+------------+ | @version | @increment | +--------------------+------------+ | 8.0.3-rc-debug-log | 11 | +--------------------+------------+
In prepared CALL
statements used
with PREPARE
and
EXECUTE
, placeholders can be used
for IN
parameters, OUT
, and
INOUT
parameters. These types of parameters can
be used as follows:
mysql>SET @increment = 10;
mysql>PREPARE s FROM 'CALL p(?, ?)';
mysql>EXECUTE s USING @version, @increment;
mysql>SELECT @version, @increment;
+--------------------+------------+ | @version | @increment | +--------------------+------------+ | 8.0.3-rc-debug-log | 11 | +--------------------+------------+
To write C programs that use the
CALL
SQL statement to execute
stored procedures that produce result sets, the
CLIENT_MULTI_RESULTS
flag must be enabled. This
is because each CALL
returns a
result to indicate the call status, in addition to any result sets
that might be returned by statements executed within the
procedure. CLIENT_MULTI_RESULTS
must also be
enabled if CALL
is used to execute
any stored procedure that contains prepared statements. It cannot
be determined when such a procedure is loaded whether those
statements will produce result sets, so it is necessary to assume
that they will.
CLIENT_MULTI_RESULTS
can be enabled when you
call mysql_real_connect()
, either
explicitly by passing the CLIENT_MULTI_RESULTS
flag itself, or implicitly by passing
CLIENT_MULTI_STATEMENTS
(which also enables
CLIENT_MULTI_RESULTS
).
CLIENT_MULTI_RESULTS
is enabled by default.
To process the result of a CALL
statement executed using
mysql_query()
or
mysql_real_query()
, use a loop
that calls mysql_next_result()
to
determine whether there are more results. For an example, see
Section 28.7.23, “C API Multiple Statement Execution Support”.
C programs can use the prepared-statement interface to execute
CALL
statements and access
OUT
and INOUT
parameters.
This is done by processing the result of a
CALL
statement using a loop that
calls mysql_stmt_next_result()
to
determine whether there are more results. For an example, see
Section 28.7.25, “C API Prepared CALL Statement Support”. Languages that
provide a MySQL interface can use prepared
CALL
statements to directly
retrieve OUT
and INOUT
procedure parameters.
Metadata changes to objects referred to by stored programs are detected and cause automatic reparsing of the affected statements when the program is next executed. For more information, see Section 8.10.3, “Caching of Prepared Statements and Stored Programs”.
DELETE
is a DML statement that
removes rows from a table.
A DELETE
statement can start with a
WITH
clause to define common table
expressions accessible within the
DELETE
. See Section 13.2.13, “WITH Syntax (Common Table Expressions)”.
DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROMtbl_name
[[AS]tbl_alias
] [PARTITION (partition_name
[,partition_name
] ...)] [WHEREwhere_condition
] [ORDER BY ...] [LIMITrow_count
]
The DELETE
statement deletes rows from
tbl_name
and returns the number of
deleted rows. To check the number of deleted rows, call the
ROW_COUNT()
function described in
Section 12.15, “Information Functions”.
The conditions in the optional WHERE
clause
identify which rows to delete. With no WHERE
clause, all rows are deleted.
where_condition
is an expression that
evaluates to true for each row to be deleted. It is specified as
described in Section 13.2.10, “SELECT Syntax”.
If the ORDER BY
clause is specified, the rows
are deleted in the order that is specified. The
LIMIT
clause places a limit on the number of
rows that can be deleted. These clauses apply to single-table
deletes, but not multi-table deletes.
DELETE [LOW_PRIORITY] [QUICK] [IGNORE]tbl_name
[.*] [,tbl_name
[.*]] ... FROMtable_references
[WHEREwhere_condition
] DELETE [LOW_PRIORITY] [QUICK] [IGNORE] FROMtbl_name
[.*] [,tbl_name
[.*]] ... USINGtable_references
[WHEREwhere_condition
]
You need the DELETE
privilege on a
table to delete rows from it. You need only the
SELECT
privilege for any columns
that are only read, such as those named in the
WHERE
clause.
When you do not need to know the number of deleted rows, the
TRUNCATE TABLE
statement is a
faster way to empty a table than a
DELETE
statement with no
WHERE
clause. Unlike
DELETE
,
TRUNCATE TABLE
cannot be used
within a transaction or if you have a lock on the table. See
Section 13.1.37, “TRUNCATE TABLE Syntax” and
Section 13.3.6, “LOCK TABLES and UNLOCK TABLES Syntax”.
The speed of delete operations may also be affected by factors discussed in Section 8.2.5.3, “Optimizing DELETE Statements”.
To ensure that a given DELETE
statement does not take too much time, the MySQL-specific
LIMIT
clause for row_count
DELETE
specifies the
maximum number of rows to be deleted. If the number of rows to
delete is larger than the limit, repeat the
DELETE
statement until the number of affected
rows is less than the LIMIT
value.
You cannot delete from a table and select from the same table in a subquery.
DELETE
supports explicit partition selection
using the PARTITION
option, which takes a list
of the comma-separated names of one or more partitions or
subpartitions (or both) from which to select rows to be dropped.
Partitions not included in the list are ignored. Given a
partitioned table t
with a partition named
p0
, executing the statement DELETE
FROM t PARTITION (p0)
has the same effect on the table
as executing ALTER
TABLE t TRUNCATE PARTITION (p0)
; in both cases, all rows
in partition p0
are dropped.
PARTITION
can be used along with a
WHERE
condition, in which case the condition is
tested only on rows in the listed partitions. For example,
DELETE FROM t PARTITION (p0) WHERE c < 5
deletes rows only from partition p0
for which
the condition c < 5
is true; rows in any
other partitions are not checked and thus not affected by the
DELETE
.
The PARTITION
option can also be used in
multiple-table DELETE
statements. You can use
up to one such option per table named in the
FROM
option.
For more information and examples, see Section 23.5, “Partition Selection”.
If you delete the row containing the maximum value for an
AUTO_INCREMENT
column, the value is not reused
for a MyISAM
or InnoDB
table. If you delete all rows in the table with DELETE
FROM
(without a
tbl_name
WHERE
clause) in
autocommit
mode, the sequence
starts over for all storage engines except
InnoDB
and MyISAM
. There are
some exceptions to this behavior for InnoDB
tables, as discussed in
Section 15.6.1.4, “AUTO_INCREMENT Handling in InnoDB”.
For MyISAM
tables, you can specify an
AUTO_INCREMENT
secondary column in a
multiple-column key. In this case, reuse of values deleted from
the top of the sequence occurs even for MyISAM
tables. See Section 3.6.9, “Using AUTO_INCREMENT”.
The DELETE
statement supports the
following modifiers:
If you specify the LOW_PRIORITY
modifier,
the server delays execution of the
DELETE
until no other clients
are reading from the table. This affects only storage engines
that use only table-level locking (such as
MyISAM
, MEMORY
, and
MERGE
).
For MyISAM
tables, if you use the
QUICK
modifier, the storage engine does not
merge index leaves during delete, which may speed up some
kinds of delete operations.
The IGNORE
modifier causes MySQL to ignore
errors during the process of deleting rows. (Errors
encountered during the parsing stage are processed in the
usual manner.) Errors that are ignored due to the use of
IGNORE
are returned as warnings. For more
information, see Comparison of the IGNORE Keyword and Strict SQL Mode.
If the DELETE
statement includes an
ORDER BY
clause, rows are deleted in the order
specified by the clause. This is useful primarily in conjunction
with LIMIT
. For example, the following
statement finds rows matching the WHERE
clause,
sorts them by timestamp_column
, and deletes the
first (oldest) one:
DELETE FROM somelog WHERE user = 'jcole' ORDER BY timestamp_column LIMIT 1;
ORDER BY
also helps to delete rows in an order
required to avoid referential integrity violations.
If you are deleting many rows from a large table, you may exceed
the lock table size for an InnoDB
table. To
avoid this problem, or simply to minimize the time that the table
remains locked, the following strategy (which does not use
DELETE
at all) might be helpful:
Select the rows not to be deleted into an empty table that has the same structure as the original table:
INSERT INTO t_copy SELECT * FROM t WHERE ... ;
Use RENAME TABLE
to atomically
move the original table out of the way and rename the copy to
the original name:
RENAME TABLE t TO t_old, t_copy TO t;
Drop the original table:
DROP TABLE t_old;
No other sessions can access the tables involved while
RENAME TABLE
executes, so the
rename operation is not subject to concurrency problems. See
Section 13.1.36, “RENAME TABLE Syntax”.
In MyISAM
tables, deleted rows are maintained
in a linked list and subsequent
INSERT
operations reuse old row
positions. To reclaim unused space and reduce file sizes, use the
OPTIMIZE TABLE
statement or the
myisamchk utility to reorganize tables.
OPTIMIZE TABLE
is easier to use,
but myisamchk is faster. See
Section 13.7.3.4, “OPTIMIZE TABLE Syntax”, and Section 4.6.4, “myisamchk — MyISAM Table-Maintenance Utility”.
The QUICK
modifier affects whether index leaves
are merged for delete operations. DELETE QUICK
is most useful for applications where index values for deleted
rows are replaced by similar index values from rows inserted
later. In this case, the holes left by deleted values are reused.
DELETE QUICK
is not useful when deleted values
lead to underfilled index blocks spanning a range of index values
for which new inserts occur again. In this case, use of
QUICK
can lead to wasted space in the index
that remains unreclaimed. Here is an example of such a scenario:
Create a table that contains an indexed
AUTO_INCREMENT
column.
Insert many rows into the table. Each insert results in an index value that is added to the high end of the index.
Delete a block of rows at the low end of the column range
using DELETE QUICK
.
In this scenario, the index blocks associated with the deleted
index values become underfilled but are not merged with other
index blocks due to the use of QUICK
. They
remain underfilled when new inserts occur, because new rows do not
have index values in the deleted range. Furthermore, they remain
underfilled even if you later use
DELETE
without
QUICK
, unless some of the deleted index values
happen to lie in index blocks within or adjacent to the
underfilled blocks. To reclaim unused index space under these
circumstances, use OPTIMIZE TABLE
.
If you are going to delete many rows from a table, it might be
faster to use DELETE QUICK
followed by
OPTIMIZE TABLE
. This rebuilds the
index rather than performing many index block merge operations.
You can specify multiple tables in a
DELETE
statement to delete rows
from one or more tables depending on the condition in the
WHERE
clause. You cannot use ORDER
BY
or LIMIT
in a multiple-table
DELETE
. The
table_references
clause lists the
tables involved in the join, as described in
Section 13.2.10.2, “JOIN Syntax”.
For the first multiple-table syntax, only matching rows from the
tables listed before the FROM
clause are
deleted. For the second multiple-table syntax, only matching rows
from the tables listed in the FROM
clause
(before the USING
clause) are deleted. The
effect is that you can delete rows from many tables at the same
time and have additional tables that are used only for searching:
DELETE t1, t2 FROM t1 INNER JOIN t2 INNER JOIN t3 WHERE t1.id=t2.id AND t2.id=t3.id;
Or:
DELETE FROM t1, t2 USING t1 INNER JOIN t2 INNER JOIN t3 WHERE t1.id=t2.id AND t2.id=t3.id;
These statements use all three tables when searching for rows to
delete, but delete matching rows only from tables
t1
and t2
.
The preceding examples use INNER JOIN
, but
multiple-table DELETE
statements
can use other types of join permitted in
SELECT
statements, such as
LEFT JOIN
. For example, to delete rows that
exist in t1
that have no match in
t2
, use a LEFT JOIN
:
DELETE t1 FROM t1 LEFT JOIN t2 ON t1.id=t2.id WHERE t2.id IS NULL;
The syntax permits .*
after each
tbl_name
for compatibility with
Access.
If you use a multiple-table DELETE
statement involving InnoDB
tables for which
there are foreign key constraints, the MySQL optimizer might
process tables in an order that differs from that of their
parent/child relationship. In this case, the statement fails and
rolls back. Instead, you should delete from a single table and
rely on the ON DELETE
capabilities that
InnoDB
provides to cause the other tables to be
modified accordingly.
If you declare an alias for a table, you must use the alias when referring to the table:
DELETE t1 FROM test AS t1, test2 WHERE ...
Table aliases in a multiple-table
DELETE
should be declared only in
the table_references
part of the
statement. Elsewhere, alias references are permitted but not alias
declarations.
Correct:
DELETE a1, a2 FROM t1 AS a1 INNER JOIN t2 AS a2 WHERE a1.id=a2.id; DELETE FROM a1, a2 USING t1 AS a1 INNER JOIN t2 AS a2 WHERE a1.id=a2.id;
Incorrect:
DELETE t1 AS a1, t2 AS a2 FROM t1 INNER JOIN t2 WHERE a1.id=a2.id; DELETE FROM t1 AS a1, t2 AS a2 USING t1 INNER JOIN t2 WHERE a1.id=a2.id;
Table aliases are also supported for single-table
DELETE
statements beginning with MySQL 8.0.16.
(Bug #89410,Bug #27455809)
DOexpr
[,expr
] ...
DO
executes the expressions but
does not return any results. In most respects,
DO
is shorthand for SELECT
, but has the
advantage that it is slightly faster when you do not care about
the result.
expr
, ...
DO
is useful primarily with
functions that have side effects, such as
RELEASE_LOCK()
.
Example: This SELECT
statement
pauses, but also produces a result set:
mysql> SELECT SLEEP(5);
+----------+
| SLEEP(5) |
+----------+
| 0 |
+----------+
1 row in set (5.02 sec)
DO
, on the other hand, pauses
without producing a result set.:
mysql> DO SLEEP(5);
Query OK, 0 rows affected (4.99 sec)
This could be useful, for example in a stored function or trigger, which prohibit statements that produce result sets.
DO
only executes expressions. It
cannot be used in all cases where SELECT
can be
used. For example, DO id FROM t1
is invalid
because it references a table.
HANDLERtbl_name
OPEN [ [AS]alias
] HANDLERtbl_name
READindex_name
{ = | <= | >= | < | > } (value1
,value2
,...) [ WHEREwhere_condition
] [LIMIT ... ] HANDLERtbl_name
READindex_name
{ FIRST | NEXT | PREV | LAST } [ WHEREwhere_condition
] [LIMIT ... ] HANDLERtbl_name
READ { FIRST | NEXT } [ WHEREwhere_condition
] [LIMIT ... ] HANDLERtbl_name
CLOSE
The HANDLER
statement provides direct access to
table storage engine interfaces. It is available for
InnoDB
and MyISAM
tables.
The HANDLER ... OPEN
statement opens a table,
making it accessible using subsequent HANDLER ...
READ
statements. This table object is not shared by
other sessions and is not closed until the session calls
HANDLER ... CLOSE
or the session terminates.
If you open the table using an alias, further references to the
open table with other HANDLER
statements must
use the alias rather than the table name. If you do not use an
alias, but open the table using a table name qualified by the
database name, further references must use the unqualified table
name. For example, for a table opened using
mydb.mytable
, further references must use
mytable
.
The first HANDLER ... READ
syntax fetches a row
where the index specified satisfies the given values and the
WHERE
condition is met. If you have a
multiple-column index, specify the index column values as a
comma-separated list. Either specify values for all the columns in
the index, or specify values for a leftmost prefix of the index
columns. Suppose that an index my_idx
includes
three columns named col_a
,
col_b
, and col_c
, in that
order. The HANDLER
statement can specify values
for all three columns in the index, or for the columns in a
leftmost prefix. For example:
HANDLER ... READ my_idx = (col_a_val,col_b_val,col_c_val) ... HANDLER ... READ my_idx = (col_a_val,col_b_val) ... HANDLER ... READ my_idx = (col_a_val) ...
To employ the HANDLER
interface to refer to a
table's PRIMARY KEY
, use the quoted identifier
`PRIMARY`
:
HANDLER tbl_name
READ `PRIMARY` ...
The second HANDLER ... READ
syntax fetches a
row from the table in index order that matches the
WHERE
condition.
The third HANDLER ... READ
syntax fetches a row
from the table in natural row order that matches the
WHERE
condition. It is faster than
HANDLER
when a full table
scan is desired. Natural row order is the order in which rows are
stored in a tbl_name
READ
index_name
MyISAM
table data file. This
statement works for InnoDB
tables as well, but
there is no such concept because there is no separate data file.
Without a LIMIT
clause, all forms of
HANDLER ... READ
fetch a single row if one is
available. To return a specific number of rows, include a
LIMIT
clause. It has the same syntax as for the
SELECT
statement. See
Section 13.2.10, “SELECT Syntax”.
HANDLER ... CLOSE
closes a table that was
opened with HANDLER ... OPEN
.
There are several reasons to use the HANDLER
interface instead of normal SELECT
statements:
HANDLER
is faster than
SELECT
:
A designated storage engine handler object is allocated
for the HANDLER ... OPEN
. The object is
reused for subsequent HANDLER
statements for that table; it need not be reinitialized
for each one.
There is less parsing involved.
There is no optimizer or query-checking overhead.
The handler interface does not have to provide a
consistent look of the data (for example,
dirty reads are
permitted), so the storage engine can use optimizations
that SELECT
does not
normally permit.
HANDLER
makes it easier to port to MySQL
applications that use a low-level ISAM
-like
interface. (See Section 15.19, “InnoDB memcached Plugin” for an
alternative way to adapt applications that use the key-value
store paradigm.)
HANDLER
enables you to traverse a database
in a manner that is difficult (or even impossible) to
accomplish with SELECT
. The
HANDLER
interface is a more natural way to
look at data when working with applications that provide an
interactive user interface to the database.
HANDLER
is a somewhat low-level statement. For
example, it does not provide consistency. That is,
HANDLER ... OPEN
does not
take a snapshot of the table, and does not
lock the table. This means that after a HANDLER ...
OPEN
statement is issued, table data can be modified (by
the current session or other sessions) and these modifications
might be only partially visible to HANDLER ...
NEXT
or HANDLER ... PREV
scans.
An open handler can be closed and marked for reopen, in which case the handler loses its position in the table. This occurs when both of the following circumstances are true:
Any session executes FLUSH
TABLES
or DDL statements on the handler's table.
The session in which the handler is open executes
non-HANDLER
statements that use tables.
TRUNCATE TABLE
for a table closes
all handlers for the table that were opened with
HANDLER OPEN
.
If a table is flushed with
FLUSH
TABLES
was opened with tbl_name
WITH READ
LOCKHANDLER
, the
handler is implicitly flushed and loses its position.
IMPORT TABLE FROMsdi_file
[,sdi_file
] ...
The IMPORT TABLE
statement imports
MyISAM
tables based on information contained in
.sdi
(Serialized Dictionary Information)
metadata files. IMPORT TABLE
requires the FILE
privilege to read
the .sdi
and table content files, and the
CREATE
privilege for the table to
be created.
Tables can be exported from one server using
mysqldump to write a file of SQL statements and
imported into another server using mysql to
process the dump file. IMPORT TABLE
provides a faster alternative using the “raw” table
files.
Prior to import, the files that provide the table content must be
placed in the appropriate schema directory for the import server,
and the .sdi
file must be located in a
directory accessible to the server. For example, the
.sdi
file can be placed in the directory
named by the secure_file_priv
system variable, or (if
secure_file_priv
is empty) in a
directory under the server data directory.
The following example describes how to export
MyISAM
tables named
employees
and managers
from
the hr
schema of one server and import them
into the hr
schema of another server. The
example uses these assumptions (to perform a similar operation on
your own system, modify the path names as appropriate):
For the export server,
export_basedir
represents its base
directory, and its data directory is
.
export_basedir
/data
For the import server,
import_basedir
represents its base
directory, and its data directory is
.
import_basedir
/data
Table files are exported from the export server into the
/tmp/export
directory and this directory
is secure (not accessible to other users).
The import server uses /tmp/mysql-files
as the directory named by its
secure_file_priv
system
variable.
To export tables from the export server, use this procedure:
Ensure a consistent snapshot by executing this statement to lock the tables so that they cannot be modified during export:
mysql> FLUSH TABLES hr.employees, hr.managers WITH READ LOCK;
While the lock is in effect, the tables can still be used, but only for read access.
At the file system level, copy the .sdi
and table content files from the hr
schema
directory to the secure export directory:
The .sdi
file is located in the
hr
schema directory, but might not have
exactly the same basename as the table name. For example,
the .sdi
files for the
employees
and
managers
tables might be named
employees_125.sdi
and
managers_238.sdi
.
For a MyISAM
table, the content files
are its .MYD
data file and
.MYI
index file.
Given those file names, the copy commands look like this:
shell>cd
shell>export_basedir
/data/hrcp employees_125.sdi /tmp/export
shell>cp managers_238.sdi /tmp/export
shell>cp employees.{MYD,MYI} /tmp/export
shell>cp managers.{MYD,MYI} /tmp/export
Unlock the tables:
mysql> UNLOCK TABLES;
To import tables into the import server, use this procedure:
The import schema must exist. If necessary, execute this statement to create it:
mysql> CREATE SCHEMA hr;
At the file system level, copy the .sdi
files to the import server
secure_file_priv
directory,
/tmp/mysql-files
. Also, copy the table
content files to the hr
schema directory:
shell>cd /tmp/export
shell>cp employees_125.sdi /tmp/mysql-files
shell>cp managers_238.sdi /tmp/mysql-files
shell>cp employees.{MYD,MYI}
shell>import_basedir
/data/hrcp managers.{MYD,MYI}
import_basedir
/data/hr
Import the tables by executing an IMPORT
TABLE
statement that names the
.sdi
files:
mysql>IMPORT TABLE FROM
'/tmp/mysql-files/employees.sdi',
'/tmp/mysql-files/managers.sdi';
The .sdi
file need not be placed in the
import server directory named by the
secure_file_priv
system variable
if that variable is empty; it can be in any directory accessible
to the server, including the schema directory for the imported
table. If the .sdi
file is placed in that
directory, however, it may be rewritten; the import operation
creates a new .sdi
file for the table, which
will overwrite the old .sdi
file if the
operation uses the same file name for the new file.
Each sdi_file
value must be a string
literal that names the .sdi
file for a table
or is a pattern that matches .sdi
files. If
the string is a pattern, any leading directory path and the
.sdi
file name suffix must be given
literally. Pattern characters are permitted only in the base name
part of the file name:
?
matches any single character
*
matches any sequence of characters,
including no characters
Using a pattern, the previous IMPORT
TABLE
statement could have been written like this
(assuming that the /tmp/mysql-files
directory
contains no other .sdi
files matching the
pattern):
IMPORT TABLE FROM '/tmp/mysql-files/*.sdi';
To interpret the location of .sdi
file path
names, the server uses the same rules for
IMPORT TABLE
as the server-side
rules for LOAD DATA
(that is, the
non-LOCAL
rules). See
Section 13.2.7, “LOAD DATA Syntax”, paying particular attention to the
rules used to interpret relative path names.
IMPORT TABLE
fails if the
.sdi
or table files cannot be located. After
importing a table, the server attempts to open it and reports as
warnings any problems detected. To attempt a repair to correct any
reported issues, use REPAIR TABLE
.
IMPORT TABLE
is not written to the
binary log.
IMPORT TABLE
applies only to
non-TEMPORARY
MyISAM
tables. It does not apply to tables created with a transactional
storage engine, tables created with
CREATE TEMPORARY
TABLE
, or views.
The table data and index files must be placed in the schema
directory for the import server prior to the import operation,
unless the table as defined on the export server uses the
DATA DIRECTORY
or INDEX
DIRECTORY
table options. In that case, modify the
import procedure using one of these alternatives before
executing the IMPORT TABLE
statement:
Put the data and index files into the same directory on the import server host as on the export server host, and create symlinks in the import server schema directory to those files.
Put the data and index files into an import server host
directory different from that on the export server host, and
create symlinks in the import server schema directory to
those files. In addition, modify the
.sdi
file to reflect the different file
locations.
Put the data and index files into the schema directory on
the import server host, and modify the
.sdi
file to remove the data and index
directory table options.
Any collation IDs stored in the .sdi
file
must refer to the same collations on the export and import
servers.
Trigger information for a table is not serialized into the table
.sdi
file, so triggers are not restored by
the import operation.
Some edits to an .sdi
file are permissible
prior to executing the IMPORT
TABLE
statement, whereas others are problematic or may
even cause the import operation to fail:
Changing the data directory and index directory table options is required if the locations of the data and index files differ between the export and import servers.
Changing the schema name is required to import the table into a different schema on the import server than on the export server.
Changing schema and table names may be required to
accommodate differences between file system case-sensitivity
semantics on the export and import servers or differences in
lower_case_table_names
settings. Changing the table names in the
.sdi
file may require renaming the
table files as well.
In some cases, changes to column definitions are permitted. Changing data types is likely to cause problems.
INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE] [INTO]tbl_name
[PARTITION (partition_name
[,partition_name
] ...)] [(col_name
[,col_name
] ...)] {VALUES | VALUE} (value_list
) [, (value_list
)] ... [ON DUPLICATE KEY UPDATEassignment_list
] INSERT [LOW_PRIORITY | DELAYED | HIGH_PRIORITY] [IGNORE] [INTO]tbl_name
[PARTITION (partition_name
[,partition_name
] ...)] SETassignment_list
[ON DUPLICATE KEY UPDATEassignment_list
] INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE] [INTO]tbl_name
[PARTITION (partition_name
[,partition_name
] ...)] [(col_name
[,col_name
] ...)] SELECT ... [ON DUPLICATE KEY UPDATEassignment_list
]value
: {expr
| DEFAULT}value_list
:value
[,value
] ...assignment
:col_name
=value
assignment_list
:assignment
[,assignment
] ...
INSERT
inserts new rows into an
existing table. The INSERT
... VALUES
and
INSERT ... SET
forms of the statement insert rows based on explicitly specified
values. The INSERT
... SELECT
form inserts rows selected from another table
or tables. INSERT
with an
ON DUPLICATE KEY UPDATE
clause enables existing
rows to be updated if a row to be inserted would cause a duplicate
value in a UNIQUE
index or PRIMARY
KEY
.
For additional information about
INSERT ...
SELECT
and
INSERT ... ON
DUPLICATE KEY UPDATE
, see
Section 13.2.6.1, “INSERT ... SELECT Syntax”, and
Section 13.2.6.2, “INSERT ... ON DUPLICATE KEY UPDATE Syntax”.
In MySQL 8.0, the DELAYED
keyword
is accepted but ignored by the server. For the reasons for this,
see Section 13.2.6.3, “INSERT DELAYED Syntax”,
Inserting into a table requires the
INSERT
privilege for the table. If
the ON DUPLICATE KEY UPDATE
clause is used and
a duplicate key causes an UPDATE
to
be performed instead, the statement requires the
UPDATE
privilege for the columns to
be updated. For columns that are read but not modified you need
only the SELECT
privilege (such as
for a column referenced only on the right hand side of an
col_name
=expr
assignment in an ON DUPLICATE KEY UPDATE
clause).
When inserting into a partitioned table, you can control which
partitions and subpartitions accept new rows. The
PARTITION
option takes a list of the
comma-separated names of one or more partitions or subpartitions
(or both) of the table. If any of the rows to be inserted by a
given INSERT
statement do not match
one of the partitions listed, the
INSERT
statement fails with the
error Found a row not matching the given partition
set. For more information and examples, see
Section 23.5, “Partition Selection”.
You can use REPLACE
instead of
INSERT
to overwrite old rows.
REPLACE
is the counterpart to
INSERT IGNORE
in
the treatment of new rows that contain unique key values that
duplicate old rows: The new rows replace the old rows rather than
being discarded. See Section 13.2.9, “REPLACE Syntax”.
tbl_name
is the table into which rows
should be inserted. Specify the columns for which the statement
provides values as follows:
Provide a parenthesized list of comma-separated column names
following the table name. In this case, a value for each named
column must be provided by the VALUES
list
or the SELECT
statement.
If you do not specify a list of column names for
INSERT ...
VALUES
or
INSERT ...
SELECT
, values for every column in the table must be
provided by the VALUES
list or the
SELECT
statement. If you do not
know the order of the columns in the table, use
DESCRIBE
to find out.
tbl_name
A SET
clause indicates columns explicitly
by name, together with the value to assign each one.
Column values can be given in several ways:
If strict SQL mode is not enabled, any column not explicitly given a value is set to its default (explicit or implicit) value. For example, if you specify a column list that does not name all the columns in the table, unnamed columns are set to their default values. Default value assignment is described in Section 11.7, “Data Type Default Values”. See also Section 1.8.3.3, “Constraints on Invalid Data”.
If strict SQL mode is enabled, an
INSERT
statement generates an
error if it does not specify an explicit value for every
column that has no default value. See
Section 5.1.11, “Server SQL Modes”.
If both the column list and the VALUES
list
are empty, INSERT
creates a row
with each column set to its default value:
INSERT INTO tbl_name
() VALUES();
If strict mode is not enabled, MySQL uses the implicit default value for any column that has no explicitly defined default. If strict mode is enabled, an error occurs if any column has no default value.
Use the keyword DEFAULT
to set a column
explicitly to its default value. This makes it easier to write
INSERT
statements that assign
values to all but a few columns, because it enables you to
avoid writing an incomplete VALUES
list
that does not include a value for each column in the table.
Otherwise, you must provide the list of column names
corresponding to each value in the VALUES
list.
If a generated column is inserted into explicitly, the only
permitted value is DEFAULT
. For information
about generated columns, see
Section 13.1.20.9, “CREATE TABLE and Generated Columns”.
In expressions, you can use
DEFAULT(
to produce the default value for column
col_name
)col_name
.
Type conversion of an expression
expr
that provides a column value
might occur if the expression data type does not match the
column data type. Conversion of a given value can result in
different inserted values depending on the column type. For
example, inserting the string '1999.0e-2'
into an INT
,
FLOAT
,
DECIMAL(10,6)
, or
YEAR
column inserts the value
1999
, 19.9921
,
19.992100
, or 1999
,
respectively. The value stored in the
INT
and
YEAR
columns is
1999
because the string-to-number
conversion looks only at as much of the initial part of the
string as may be considered a valid integer or year. For the
FLOAT
and
DECIMAL
columns, the
string-to-number conversion considers the entire string a
valid numeric value.
An expression expr
can refer to any
column that was set earlier in a value list. For example, you
can do this because the value for col2
refers to col1
, which has previously been
assigned:
INSERT INTO tbl_name
(col1,col2) VALUES(15,col1*2);
But the following is not legal, because the value for
col1
refers to col2
,
which is assigned after col1
:
INSERT INTO tbl_name
(col1,col2) VALUES(col2*2,15);
An exception occurs for columns that contain
AUTO_INCREMENT
values. Because
AUTO_INCREMENT
values are generated after
other value assignments, any reference to an
AUTO_INCREMENT
column in the assignment
returns a 0
.
INSERT
statements that use
VALUES
syntax can insert multiple rows. To do
this, include multiple lists of comma-separated column values,
with lists enclosed within parentheses and separated by commas.
Example:
INSERT INTO tbl_name
(a,b,c) VALUES(1,2,3),(4,5,6),(7,8,9);
Each values list must contain exactly as many values as are to be inserted per row. The following statement is invalid because it contains one list of nine values, rather than three lists of three values each:
INSERT INTO tbl_name
(a,b,c) VALUES(1,2,3,4,5,6,7,8,9);
VALUE
is a synonym for
VALUES
in this context. Neither implies
anything about the number of values lists, nor about the number of
values per list. Either may be used whether there is a single
values list or multiple lists, and regardless of the number of
values per list.
The affected-rows value for an
INSERT
can be obtained using the
ROW_COUNT()
SQL function or the
mysql_affected_rows()
C API
function. See Section 12.15, “Information Functions”, and
Section 28.7.7.1, “mysql_affected_rows()”.
If you use an INSERT ...
VALUES
statement with multiple value lists or
INSERT ...
SELECT
, the statement returns an information string in
this format:
Records:N1
Duplicates:N2
Warnings:N3
If you are using the C API, the information string can be obtained
by invoking the mysql_info()
function. See Section 28.7.7.36, “mysql_info()”.
Records
indicates the number of rows processed
by the statement. (This is not necessarily the number of rows
actually inserted because Duplicates
can be
nonzero.) Duplicates
indicates the number of
rows that could not be inserted because they would duplicate some
existing unique index value. Warnings
indicates
the number of attempts to insert column values that were
problematic in some way. Warnings can occur under any of the
following conditions:
Inserting NULL
into a column that has been
declared NOT NULL
. For multiple-row
INSERT
statements or
INSERT INTO ...
SELECT
statements, the column is set to the implicit
default value for the column data type. This is
0
for numeric types, the empty string
(''
) for string types, and the
“zero” value for date and time types.
INSERT INTO ...
SELECT
statements are handled the same way as
multiple-row inserts because the server does not examine the
result set from the SELECT
to
see whether it returns a single row. (For a single-row
INSERT
, no warning occurs when
NULL
is inserted into a NOT
NULL
column. Instead, the statement fails with an
error.)
Setting a numeric column to a value that lies outside the column's range. The value is clipped to the closest endpoint of the range.
Assigning a value such as '10.34 a'
to a
numeric column. The trailing nonnumeric text is stripped off
and the remaining numeric part is inserted. If the string
value has no leading numeric part, the column is set to
0
.
Inserting a string into a string column
(CHAR
,
VARCHAR
,
TEXT
, or
BLOB
) that exceeds the column's
maximum length. The value is truncated to the column's maximum
length.
Inserting a value into a date or time column that is illegal for the data type. The column is set to the appropriate zero value for the type.
For INSERT
examples involving
AUTO_INCREMET
column values, see
Section 3.6.9, “Using AUTO_INCREMENT”.
If INSERT
inserts a row into a
table that has an AUTO_INCREMENT
column,
you can find the value used for that column by using the
LAST_INSERT_ID()
SQL function
or the mysql_insert_id()
C API
function.
These two functions do not always behave identically. The
behavior of INSERT
statements
with respect to AUTO_INCREMENT
columns is
discussed further in
Section 12.15, “Information Functions”, and
Section 28.7.7.38, “mysql_insert_id()”.
The INSERT
statement supports the
following modifiers:
If you use the LOW_PRIORITY
modifier,
execution of the INSERT
is
delayed until no other clients are reading from the table.
This includes other clients that began reading while existing
clients are reading, and while the INSERT
LOW_PRIORITY
statement is waiting. It is possible,
therefore, for a client that issues an INSERT
LOW_PRIORITY
statement to wait for a very long time.
LOW_PRIORITY
affects only storage engines
that use only table-level locking (such as
MyISAM
, MEMORY
, and
MERGE
).
LOW_PRIORITY
should normally not be used
with MyISAM
tables because doing so
disables concurrent inserts. See
Section 8.11.3, “Concurrent Inserts”.
If you specify HIGH_PRIORITY
, it overrides
the effect of the
--low-priority-updates
option
if the server was started with that option. It also causes
concurrent inserts not to be used. See
Section 8.11.3, “Concurrent Inserts”.
HIGH_PRIORITY
affects only storage engines
that use only table-level locking (such as
MyISAM
, MEMORY
, and
MERGE
).
If you use the IGNORE
modifier, errors that
occur while executing the
INSERT
statement are ignored.
For example, without IGNORE
, a row that
duplicates an existing UNIQUE
index or
PRIMARY KEY
value in the table causes a
duplicate-key error and the statement is aborted. With
IGNORE
, the row is discarded and no error
occurs. Ignored errors generate warnings instead.
IGNORE
has a similar effect on inserts into
partitioned tables where no partition matching a given value
is found. Without IGNORE
, such
INSERT
statements are aborted
with an error. When
INSERT
IGNORE
is used, the insert operation fails silently
for rows containing the unmatched value, but inserts rows that
are matched. For an example, see
Section 23.2.2, “LIST Partitioning”.
Data conversions that would trigger errors abort the statement
if IGNORE
is not specified. With
IGNORE
, invalid values are adjusted to the
closest values and inserted; warnings are produced but the
statement does not abort. You can determine with the
mysql_info()
C API function
how many rows were actually inserted into the table.
For more information, see Comparison of the IGNORE Keyword and Strict SQL Mode.
If you specify ON DUPLICATE KEY UPDATE
, and
a row is inserted that would cause a duplicate value in a
UNIQUE
index or PRIMARY
KEY
, an UPDATE
of the
old row occurs. The affected-rows value per row is 1 if the
row is inserted as a new row, 2 if an existing row is updated,
and 0 if an existing row is set to its current values. If you
specify the CLIENT_FOUND_ROWS
flag to the
mysql_real_connect()
C API
function when connecting to mysqld, the
affected-rows value is 1 (not 0) if an existing row is set to
its current values. See Section 13.2.6.2, “INSERT ... ON DUPLICATE KEY UPDATE Syntax”.
INSERT DELAYED
was deprecated
in MySQL 5.6, and is scheduled for eventual removal. In MySQL
8.0, the DELAYED
modifier is
accepted but ignored. Use INSERT
(without
DELAYED
) instead. See
Section 13.2.6.3, “INSERT DELAYED Syntax”.
An INSERT
statement affecting a partitioned
table using a storage engine such as
MyISAM
that employs table-level locks
locks only those partitions into which rows are actually inserted.
(For storage engines such as InnoDB
that employ row-level locking, no locking of partitions takes
place.) For more information, see
Partitioning and Locking.
INSERT [LOW_PRIORITY | HIGH_PRIORITY] [IGNORE] [INTO]tbl_name
[PARTITION (partition_name
[,partition_name
] ...)] [(col_name
[,col_name
] ...)] SELECT ... [ON DUPLICATE KEY UPDATEassignment_list
]value
: {expr
| DEFAULT}assignment
:col_name
=value
assignment_list
:assignment
[,assignment
] ...
With INSERT ...
SELECT
, you can quickly insert many rows into a table
from the result of a SELECT
statement, which can select from one or many tables. For
example:
INSERT INTO tbl_temp2 (fld_id) SELECT tbl_temp1.fld_order_id FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;
The following conditions hold for
INSERT ...
SELECT
statements:
Specify IGNORE
to ignore rows that would
cause duplicate-key violations.
The target table of the
INSERT
statement may appear
in the FROM
clause of the
SELECT
part of the query.
However, you cannot insert into a table and select from the
same table in a subquery.
When selecting from and inserting into the same table, MySQL
creates an internal temporary table to hold the rows from
the SELECT
and then inserts
those rows into the target table. However, you cannot use
INSERT INTO t ... SELECT ... FROM t
when
t
is a TEMPORARY
table, because TEMPORARY
tables cannot be
referred to twice in the same statement. See
Section 8.4.4, “Internal Temporary Table Use in MySQL”, and
Section B.6.6.2, “TEMPORARY Table Problems”.
AUTO_INCREMENT
columns work as usual.
To ensure that the binary log can be used to re-create the
original tables, MySQL does not permit concurrent inserts
for INSERT
... SELECT
statements (see
Section 8.11.3, “Concurrent Inserts”).
To avoid ambiguous column reference problems when the
SELECT
and the
INSERT
refer to the same
table, provide a unique alias for each table used in the
SELECT
part, and qualify
column names in that part with the appropriate alias.
You can explicitly select which partitions or subpartitions (or
both) of the source or target table (or both) are to be used
with a PARTITION
option following the name of
the table. When PARTITION
is used with the
name of the source table in the
SELECT
portion of the statement,
rows are selected only from the partitions or subpartitions
named in its partition list. When PARTITION
is used with the name of the target table for the
INSERT
portion of the statement,
it must be possible to insert all rows selected into the
partitions or subpartitions named in the partition list
following the option. Otherwise, the INSERT ...
SELECT
statement fails. For more information and
examples, see Section 23.5, “Partition Selection”.
For INSERT
... SELECT
statements, see
Section 13.2.6.2, “INSERT ... ON DUPLICATE KEY UPDATE Syntax” for conditions under which
the SELECT
columns can be
referred to in an ON DUPLICATE KEY UPDATE
clause.
The order in which a SELECT
statement with no ORDER BY
clause returns
rows is nondeterministic. This means that, when using
replication, there is no guarantee that such a
SELECT
returns rows in the same
order on the master and the slave, which can lead to
inconsistencies between them. To prevent this from occurring,
always write INSERT ... SELECT
statements
that are to be replicated using an ORDER BY
clause that produces the same row order on the master and the
slave. See also Section 17.4.1.18, “Replication and LIMIT”.
Due to this issue,
INSERT ...
SELECT ON DUPLICATE KEY UPDATE
and
INSERT IGNORE ...
SELECT
statements are flagged as unsafe for
statement-based replication. Such statements produce a warning
in the error log when using statement-based mode and are written
to the binary log using the row-based format when using
MIXED
mode. (Bug #11758262, Bug #50439)
See also Section 17.2.1.1, “Advantages and Disadvantages of Statement-Based and Row-Based Replication”.
An INSERT ... SELECT
statement affecting
partitioned tables using a storage engine such as
MyISAM
that employs table-level
locks locks all partitions of the target table; however, only
those partitions that are actually read from the source table
are locked. (This does not occur with tables using storage
engines such as InnoDB
that employ
row-level locking.) For more information, see
Partitioning and Locking.
If you specify an ON DUPLICATE KEY UPDATE
clause and a row to be inserted would cause a duplicate value in
a UNIQUE
index or PRIMARY
KEY
, an UPDATE
of the
old row occurs. For example, if column a
is
declared as UNIQUE
and contains the value
1
, the following two statements have similar
effect:
INSERT INTO t1 (a,b,c) VALUES (1,2,3) ON DUPLICATE KEY UPDATE c=c+1; UPDATE t1 SET c=c+1 WHERE a=1;
(The effects are not identical for an InnoDB
table where a
is an auto-increment column.
With an auto-increment column, an INSERT
statement increases the auto-increment value but
UPDATE
does not.)
If column b
is also unique, the
INSERT
is equivalent to this
UPDATE
statement instead:
UPDATE t1 SET c=c+1 WHERE a=1 OR b=2 LIMIT 1;
If a=1 OR b=2
matches several rows, only
one row is updated. In general, you should
try to avoid using an ON DUPLICATE KEY UPDATE
clause on tables with multiple unique indexes.
With ON DUPLICATE KEY UPDATE
, the
affected-rows value per row is 1 if the row is inserted as a new
row, 2 if an existing row is updated, and 0 if an existing row
is set to its current values. If you specify the
CLIENT_FOUND_ROWS
flag to the
mysql_real_connect()
C API
function when connecting to mysqld, the
affected-rows value is 1 (not 0) if an existing row is set to
its current values.
If a table contains an AUTO_INCREMENT
column
and INSERT
... ON DUPLICATE KEY UPDATE
inserts or updates a row,
the LAST_INSERT_ID()
function
returns the AUTO_INCREMENT
value.
The ON DUPLICATE KEY UPDATE
clause can
contain multiple column assignments, separated by commas.
In assignment value expressions in the ON DUPLICATE KEY
UPDATE
clause, you can use the
VALUES(
function to refer to column values from the
col_name
)INSERT
portion of the
INSERT ...
ON DUPLICATE KEY UPDATE
statement. In other words,
VALUES(
in the col_name
)ON DUPLICATE KEY UPDATE
clause refers
to the value of col_name
that would
be inserted, had no duplicate-key conflict occurred. This
function is especially useful in multiple-row inserts. The
VALUES()
function is meaningful
only in the ON DUPLICATE KEY UPDATE
clause or
INSERT
statements and returns
NULL
otherwise. Example:
INSERT INTO t1 (a,b,c) VALUES (1,2,3),(4,5,6) ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);
That statement is identical to the following two statements:
INSERT INTO t1 (a,b,c) VALUES (1,2,3) ON DUPLICATE KEY UPDATE c=3; INSERT INTO t1 (a,b,c) VALUES (4,5,6) ON DUPLICATE KEY UPDATE c=9;
For INSERT
... SELECT
statements, these rules apply regarding
acceptable forms of SELECT
query expressions
that you can refer to in an ON DUPLICATE KEY
UPDATE
clause:
References to columns from queries on a single table, which may be a derived table.
References to columns from queries on a join over multiple tables.
References to columns from DISTINCT
queries.
References to columns in other tables, as long as the
SELECT
does not use
GROUP BY
. One side effect is that you
must qualify references to nonunique column names.
References to columns from a
UNION
are not supported. To work
around this restriction, rewrite the
UNION
as a derived table so that
its rows can be treated as a single-table result set. For
example, this statement produces an error:
INSERT INTO t1 (a, b) SELECT c, d FROM t2 UNION SELECT e, f FROM t3 ON DUPLICATE KEY UPDATE b = b + c;
Instead, use an equivalent statement that rewrites the
UNION
as a derived table:
INSERT INTO t1 (a, b) SELECT * FROM (SELECT c, d FROM t2 UNION SELECT e, f FROM t3) AS dt ON DUPLICATE KEY UPDATE b = b + c;
The technique of rewriting a query as a derived table also
enables references to columns from GROUP BY
queries.
Because the results of
INSERT ...
SELECT
statements depend on the ordering of rows from
the SELECT
and this order cannot
always be guaranteed, it is possible when logging
INSERT ...
SELECT ON DUPLICATE KEY UPDATE
statements for the
master and the slave to diverge. Thus,
INSERT ...
SELECT ON DUPLICATE KEY UPDATE
statements are flagged
as unsafe for statement-based replication. Such statements
produce a warning in the error log when using statement-based
mode and are written to the binary log using the row-based
format when using MIXED
mode. An
INSERT ...
ON DUPLICATE KEY UPDATE
statement against a table
having more than one unique or primary key is also marked as
unsafe. (Bug #11765650, Bug #58637)
See also Section 17.2.1.1, “Advantages and Disadvantages of Statement-Based and Row-Based Replication”.
An INSERT ... ON DUPLICATE KEY UPDATE
on a
partitioned table using a storage engine such as
MyISAM
that employs table-level
locks locks any partitions of the table in which a partitioning
key column is updated. (This does not occur with tables using
storage engines such as InnoDB
that
employ row-level locking.) For more information, see
Partitioning and Locking.
INSERT DELAYED ...
The DELAYED
option for the
INSERT
statement is a MySQL
extension to standard SQL. In previous versions of MySQL, it can
be used for certain kinds of tables (such as
MyISAM
), such that when a client uses
INSERT DELAYED
, it gets an okay
from the server at once, and the row is queued to be inserted
when the table is not in use by any other thread.
DELAYED
inserts and replaces were deprecated
in MySQL 5.6. In MySQL 8.0,
DELAYED
is not supported. The server
recognizes but ignores the DELAYED
keyword,
handles the insert as a nondelayed insert, and generates an
ER_WARN_LEGACY_SYNTAX_CONVERTED
warning
(“INSERT DELAYED is no longer supported. The statement was
converted to INSERT”). The DELAYED
keyword is scheduled for removal in a future release.
LOAD DATA [LOW_PRIORITY | CONCURRENT] [LOCAL] INFILE 'file_name
' [REPLACE | IGNORE] INTO TABLEtbl_name
[PARTITION (partition_name
[,partition_name
] ...)] [CHARACTER SETcharset_name
] [{FIELDS | COLUMNS} [TERMINATED BY 'string
'] [[OPTIONALLY] ENCLOSED BY 'char
'] [ESCAPED BY 'char
'] ] [LINES [STARTING BY 'string
'] [TERMINATED BY 'string
'] ] [IGNOREnumber
{LINES | ROWS}] [(col_name_or_user_var
[,col_name_or_user_var
] ...)] [SETcol_name
={expr
| DEFAULT}, [,col_name
={expr
| DEFAULT}] ...]
The LOAD DATA
statement reads rows
from a text file into a table at a very high speed.
LOAD DATA
is the complement of
SELECT ... INTO
OUTFILE
. (See Section 13.2.10.1, “SELECT ... INTO Syntax”.) To write
data from a table to a file, use
SELECT ... INTO
OUTFILE
. To read the file back into a table, use
LOAD DATA
. The syntax of the
FIELDS
and LINES
clauses is
the same for both statements.
You can also load data files by using the
mysqlimport utility; see
Section 4.5.5, “mysqlimport — A Data Import Program”. mysqlimport
operates by sending a LOAD DATA
statement to the server.
For more information about the efficiency of
INSERT
versus
LOAD DATA
and speeding up
LOAD DATA
, see
Section 8.2.5.1, “Optimizing INSERT Statements”.
LOAD DATA
supports explicit
partition selection using the PARTITION
option with a list of one or more comma-separated names of
partitions, subpartitions, or both. When this option is used, if
any rows from the file cannot be inserted into any of the
partitions or subpartitions named in the list, the statement
fails with the error Found a row not matching the
given partition set. For more information and
examples, see Section 23.5, “Partition Selection”.
For partitioned tables using storage engines that employ table
locks, such as MyISAM
,
LOAD DATA
cannot prune any
partition locks. This does not apply to tables using storage
engines that employ row-level locking, such as
InnoDB
. For more information, see
Partitioning and Locking.
The file name must be given as a literal string. On Windows,
specify backslashes in path names as forward slashes or doubled
backslashes. The
character_set_filesystem
system
variable controls the interpretation of the file name character
set.
The server uses the character set indicated by the
character_set_database
system
variable to interpret the information in the file.
SET NAMES
and the setting of
character_set_client
do not
affect interpretation of input. If the contents of the input
file use a character set that differs from the default, it is
usually preferable to specify the character set of the file by
using the CHARACTER SET
clause. A character
set of binary
specifies “no
conversion.”
LOAD DATA
interprets all fields
in the file as having the same character set, regardless of the
data types of the columns into which field values are loaded.
For proper interpretation of file contents, you must ensure that
it was written with the correct character set. For example, if
you write a data file with mysqldump -T or by
issuing a SELECT
... INTO OUTFILE
statement in
mysql, be sure to use a
--default-character-set
option so that output
is written in the character set to be used when the file is
loaded with LOAD DATA
.
It is not possible to load data files that use the
ucs2
, utf16
,
utf16le
, or utf32
character set.
If you use the LOW_PRIORITY
modifier,
execution of the LOAD DATA
statement is delayed until no other clients are reading from the
table. This affects only storage engines that use only
table-level locking (such as MyISAM
,
MEMORY
, and MERGE
).
If you specify the CONCURRENT
modifier with a
MyISAM
table that satisfies the condition for
concurrent inserts (that is, it contains no free blocks in the
middle), other threads can retrieve data from the table while
LOAD DATA
is executing. This
modifier affects the performance of LOAD
DATA
a bit, even if no other thread is using the table
at the same time.
For information about LOAD DATA
in relation to replication, see
Section 17.4.1.19, “Replication and LOAD DATA”.
The LOCAL
modifier affects expected location
of the file and error handling, as described later.
LOCAL
works only if your server and your
client both have been configured to permit it. For example, if
mysqld was started with the
local_infile
system variable
disabled, LOCAL
does not work. See
Section 6.1.6, “Security Issues with LOAD DATA LOCAL”.
The LOCAL
modifier affects where the file is
expected to be found:
If LOCAL
is specified, the file is read
by the client program on the client host and sent to the
server. The file can be given as a full path name to specify
its exact location. If given as a relative path name, the
name is interpreted relative to the directory in which the
client program was started.
When using LOCAL
with
LOAD DATA
, a copy of the file
is created in the directory where the MySQL server stores
temporary files. See Section B.6.3.5, “Where MySQL Stores Temporary Files”. Lack
of sufficient space for the copy in this directory can cause
the LOAD DATA
LOCAL
statement to fail.
If LOCAL
is not specified, the file must
be located on the server host and is read directly by the
server. The server uses the following rules to locate the
file:
If the file name is an absolute path name, the server uses it as given.
If the file name is a relative path name with one or more leading components, the server searches for the file relative to the server's data directory.
If a file name with no leading components is given, the server looks for the file in the database directory of the default database.
In the non-LOCAL
case, these rules mean that
a file named as ./myfile.txt
is read from
the server's data directory, whereas the file named as
myfile.txt
is read from the database
directory of the default database. For example, if
db1
is the default database, the following
LOAD DATA
statement reads the
file data.txt
from the database directory
for db1
, even though the statement explicitly
loads the file into a table in the db2
database:
LOAD DATA INFILE 'data.txt' INTO TABLE db2.my_table;
The server also uses the non-LOCAL
rules to
locate .sdi
files for the
IMPORT TABLE
statement.
Non-LOCAL
load operations read text files
located on the server. For security reasons, such operations
require that you have the FILE
privilege. See Section 6.2.1, “Privileges Provided by MySQL”. Also,
non-LOCAL
load operations are subject to the
secure_file_priv
system
variable setting. If the variable value is a nonempty directory
name, the file to be loaded must be located in that directory.
If the variable value is empty (which is insecure), the file
need only be readable by the server.
Using LOCAL
is a bit slower than letting the
server access the files directly, because the file contents must
be sent over the connection by the client to the server. On the
other hand, you do not need the
FILE
privilege to load local
files.
LOCAL
also affects error handling:
With LOAD DATA
,
data-interpretation and duplicate-key errors terminate the
operation.
With LOAD DATA
LOCAL
, data-interpretation and duplicate-key
errors become warnings and the operation continues because
the server has no way to stop transmission of the file in
the middle of the operation. For duplicate-key errors, this
is the same as if IGNORE
is specified.
IGNORE
is explained further later in this
section.
The REPLACE
and IGNORE
modifiers control handling of input rows that duplicate existing
rows on unique key values:
If you specify REPLACE
, input rows
replace existing rows. In other words, rows that have the
same value for a primary key or unique index as an existing
row. See Section 13.2.9, “REPLACE Syntax”.
If you specify IGNORE
, rows that
duplicate an existing row on a unique key value are
discarded. For more information, see
Comparison of the IGNORE Keyword and Strict SQL Mode.
If you do not specify either modifier, the behavior depends
on whether the LOCAL
modifier is
specified. Without LOCAL
, an error occurs
when a duplicate key value is found, and the rest of the
text file is ignored. With LOCAL
, the
default behavior is the same as if IGNORE
is specified; this is because the server has no way to stop
transmission of the file in the middle of the operation.
To ignore foreign key constraints during the load operation,
execute a SET foreign_key_checks = 0
statement before executing LOAD
DATA
.
If you use LOAD DATA
on an empty
MyISAM
table, all nonunique indexes are
created in a separate batch (as for REPAIR
TABLE
). Normally, this makes LOAD
DATA
much faster when you have many indexes. In some
extreme cases, you can create the indexes even faster by turning
them off with ALTER
TABLE ... DISABLE KEYS
before loading the file into
the table and using
ALTER TABLE ...
ENABLE KEYS
to re-create the indexes after loading the
file. See Section 8.2.5.1, “Optimizing INSERT Statements”.
For both the LOAD DATA
and
SELECT ... INTO
OUTFILE
statements, the syntax of the
FIELDS
and LINES
clauses
is the same. Both clauses are optional, but
FIELDS
must precede LINES
if both are specified.
If you specify a FIELDS
clause, each of its
subclauses (TERMINATED BY
,
[OPTIONALLY] ENCLOSED BY
, and
ESCAPED BY
) is also optional, except that you
must specify at least one of them. Arguments to these clauses
are permitted to contain only ASCII characters.
If you specify no FIELDS
or
LINES
clause, the defaults are the same as if
you had written this:
FIELDS TERMINATED BY '\t' ENCLOSED BY '' ESCAPED BY '\\' LINES TERMINATED BY '\n' STARTING BY ''
Backslash is the MySQL escape character within strings in SQL
statements. Thus, to specify a literal backslash, you must
specify two backslashes for the value to be interpreted as a
single backslash. The escape sequences '\t'
and '\n'
specify tab and newline characters,
respectively.
In other words, the defaults cause LOAD
DATA
to act as follows when reading input:
Look for line boundaries at newlines.
Do not skip any line prefix.
Break lines into fields at tabs.
Do not expect fields to be enclosed within any quoting characters.
Interpret characters preceded by the escape character
\
as escape sequences. For example,
\t
, \n
, and
\\
signify tab, newline, and backslash,
respectively. See the discussion of FIELDS ESCAPED
BY
later for the full list of escape sequences.
Conversely, the defaults cause
SELECT ... INTO
OUTFILE
to act as follows when writing output:
Write tabs between fields.
Do not enclose fields within any quoting characters.
Use \
to escape instances of tab,
newline, or \
that occur within field
values.
Write newlines at the ends of lines.
For a text file generated on a Windows system, proper file
reading might require LINES TERMINATED BY
'\r\n'
because Windows programs typically use two
characters as a line terminator. Some programs, such as
WordPad, might use \r
as
a line terminator when writing files. To read such files, use
LINES TERMINATED BY '\r'
.
If all the input lines have a common prefix that you want to
ignore, you can use LINES STARTING BY
'
to skip the
prefix and anything before it. If a line
does not include the prefix, the entire line is skipped. Suppose
that you issue the following statement:
prefix_string
'
LOAD DATA INFILE '/tmp/test.txt' INTO TABLE test FIELDS TERMINATED BY ',' LINES STARTING BY 'xxx';
If the data file looks like this:
xxx"abc",1 something xxx"def",2 "ghi",3
The resulting rows will be ("abc",1)
and
("def",2)
. The third row in the file is
skipped because it does not contain the prefix.
The IGNORE
option can be used to ignore lines at the start
of the file. For example, you can use number
LINESIGNORE 1
LINES
to skip an initial header line containing column
names:
LOAD DATA INFILE '/tmp/test.txt' INTO TABLE test IGNORE 1 LINES;
When you use SELECT
... INTO OUTFILE
in tandem with
LOAD DATA
to write data from a
database into a file and then read the file back into the
database later, the field- and line-handling options for both
statements must match. Otherwise, LOAD
DATA
will not interpret the contents of the file
properly. Suppose that you use
SELECT ... INTO
OUTFILE
to write a file with fields delimited by
commas:
SELECT * INTO OUTFILE 'data.txt' FIELDS TERMINATED BY ',' FROM table2;
To read the comma-delimited file, the correct statement would be:
LOAD DATA INFILE 'data.txt' INTO TABLE table2 FIELDS TERMINATED BY ',';
If instead you tried to read the file with the statement shown
following, it wouldn't work because it instructs
LOAD DATA
to look for tabs
between fields:
LOAD DATA INFILE 'data.txt' INTO TABLE table2 FIELDS TERMINATED BY '\t';
The likely result is that each input line would be interpreted as a single field.
LOAD DATA
can be used to read
files obtained from external sources. For example, many programs
can export data in comma-separated values (CSV) format, such
that lines have fields separated by commas and enclosed within
double quotation marks, with an initial line of column names. If
the lines in such a file are terminated by carriage
return/newline pairs, the statement shown here illustrates the
field- and line-handling options you would use to load the file:
LOAD DATA INFILE 'data.txt' INTO TABLE tbl_name
FIELDS TERMINATED BY ',' ENCLOSED BY '"'
LINES TERMINATED BY '\r\n'
IGNORE 1 LINES;
If the input values are not necessarily enclosed within
quotation marks, use OPTIONALLY
before the
ENCLOSED BY
option.
Any of the field- or line-handling options can specify an empty
string (''
). If not empty, the
FIELDS [OPTIONALLY] ENCLOSED BY
and
FIELDS ESCAPED BY
values must be a single
character. The FIELDS TERMINATED BY
,
LINES STARTING BY
, and LINES
TERMINATED BY
values can be more than one character.
For example, to write lines that are terminated by carriage
return/linefeed pairs, or to read a file containing such lines,
specify a LINES TERMINATED BY '\r\n'
clause.
To read a file containing jokes that are separated by lines
consisting of %%
, you can do this
CREATE TABLE jokes (a INT NOT NULL AUTO_INCREMENT PRIMARY KEY, joke TEXT NOT NULL); LOAD DATA INFILE '/tmp/jokes.txt' INTO TABLE jokes FIELDS TERMINATED BY '' LINES TERMINATED BY '\n%%\n' (joke);
FIELDS [OPTIONALLY] ENCLOSED BY
controls
quoting of fields. For output
(SELECT ... INTO
OUTFILE
), if you omit the word
OPTIONALLY
, all fields are enclosed by the
ENCLOSED BY
character. An example of such
output (using a comma as the field delimiter) is shown here:
"1","a string","100.20" "2","a string containing a , comma","102.20" "3","a string containing a \" quote","102.20" "4","a string containing a \", quote and comma","102.20"
If you specify OPTIONALLY
, the
ENCLOSED BY
character is used only to enclose
values from columns that have a string data type (such as
CHAR
,
BINARY
,
TEXT
, or
ENUM
):
1,"a string",100.20 2,"a string containing a , comma",102.20 3,"a string containing a \" quote",102.20 4,"a string containing a \", quote and comma",102.20
Occurrences of the ENCLOSED BY
character
within a field value are escaped by prefixing them with the
ESCAPED BY
character. Also, if you specify an
empty ESCAPED BY
value, it is possible to
inadvertently generate output that cannot be read properly by
LOAD DATA
. For example, the
preceding output just shown would appear as follows if the
escape character is empty. Observe that the second field in the
fourth line contains a comma following the quote, which
(erroneously) appears to terminate the field:
1,"a string",100.20 2,"a string containing a , comma",102.20 3,"a string containing a " quote",102.20 4,"a string containing a ", quote and comma",102.20
For input, the ENCLOSED BY
character, if
present, is stripped from the ends of field values. (This is
true regardless of whether OPTIONALLY
is
specified; OPTIONALLY
has no effect on input
interpretation.) Occurrences of the ENCLOSED
BY
character preceded by the ESCAPED
BY
character are interpreted as part of the current
field value.
If the field begins with the ENCLOSED BY
character, instances of that character are recognized as
terminating a field value only if followed by the field or line
TERMINATED BY
sequence. To avoid ambiguity,
occurrences of the ENCLOSED BY
character
within a field value can be doubled and are interpreted as a
single instance of the character. For example, if
ENCLOSED BY '"'
is specified, quotation marks
are handled as shown here:
"The ""BIG"" boss" -> The "BIG" boss The "BIG" boss -> The "BIG" boss The ""BIG"" boss -> The ""BIG"" boss
FIELDS ESCAPED BY
controls how to read or
write special characters:
For input, if the FIELDS ESCAPED BY
character is not empty, occurrences of that character are
stripped and the following character is taken literally as
part of a field value. Some two-character sequences that are
exceptions, where the first character is the escape
character. These sequences are shown in the following table
(using \
for the escape character). The
rules for NULL
handling are described
later in this section.
For more information about \
-escape
syntax, see Section 9.1.1, “String Literals”.
If the FIELDS ESCAPED BY
character is
empty, escape-sequence interpretation does not occur.
For output, if the FIELDS ESCAPED BY
character is not empty, it is used to prefix the following
characters on output:
The FIELDS ESCAPED BY
character.
The FIELDS [OPTIONALLY] ENCLOSED BY
character.
The first character of the FIELDS TERMINATED
BY
and LINES TERMINATED BY
values, if the ENCLOSED BY
character
is empty or unspecified.
ASCII 0
(what is actually written
following the escape character is ASCII
0
, not a zero-valued byte).
If the FIELDS ESCAPED BY
character is
empty, no characters are escaped and NULL
is output as NULL
, not
\N
. It is probably not a good idea to
specify an empty escape character, particularly if field
values in your data contain any of the characters in the
list just given.
In certain cases, field- and line-handling options interact:
If LINES TERMINATED BY
is an empty string
and FIELDS TERMINATED BY
is nonempty,
lines are also terminated with FIELDS TERMINATED
BY
.
If the FIELDS TERMINATED BY
and
FIELDS ENCLOSED BY
values are both empty
(''
), a fixed-row (nondelimited) format
is used. With fixed-row format, no delimiters are used
between fields (but you can still have a line terminator).
Instead, column values are read and written using a field
width wide enough to hold all values in the field. For
TINYINT
,
SMALLINT
,
MEDIUMINT
,
INT
, and
BIGINT
, the field widths are
4, 6, 8, 11, and 20, respectively, no matter what the
declared display width is.
LINES TERMINATED BY
is still used to
separate lines. If a line does not contain all fields, the
rest of the columns are set to their default values. If you
do not have a line terminator, you should set this to
''
. In this case, the text file must
contain all fields for each row.
Fixed-row format also affects handling of
NULL
values, as described later.
Fixed-size format does not work if you are using a multibyte character set.
Handling of NULL
values varies according to
the FIELDS
and LINES
options in use:
For the default FIELDS
and
LINES
values, NULL
is
written as a field value of \N
for
output, and a field value of \N
is read
as NULL
for input (assuming that the
ESCAPED BY
character is
\
).
If FIELDS ENCLOSED BY
is not empty, a
field containing the literal word NULL
as
its value is read as a NULL
value. This
differs from the word NULL
enclosed
within FIELDS ENCLOSED BY
characters,
which is read as the string 'NULL'
.
If FIELDS ESCAPED BY
is empty,
NULL
is written as the word
NULL
.
With fixed-row format (which is used when FIELDS
TERMINATED BY
and FIELDS ENCLOSED
BY
are both empty), NULL
is
written as an empty string. This causes both
NULL
values and empty strings in the
table to be indistinguishable when written to the file
because both are written as empty strings. If you need to be
able to tell the two apart when reading the file back in,
you should not use fixed-row format.
An attempt to load NULL
into a NOT
NULL
column causes assignment of the implicit default
value for the column's data type and a warning, or an error in
strict SQL mode. Implicit default values are discussed in
Section 11.7, “Data Type Default Values”.
Some cases are not supported by LOAD
DATA
:
Fixed-size rows (FIELDS TERMINATED BY
and
FIELDS ENCLOSED BY
both empty) and
BLOB
or
TEXT
columns.
If you specify one separator that is the same as or a prefix
of another, LOAD DATA
cannot
interpret the input properly. For example, the following
FIELDS
clause would cause problems:
FIELDS TERMINATED BY '"' ENCLOSED BY '"'
If FIELDS ESCAPED BY
is empty, a field
value that contains an occurrence of FIELDS
ENCLOSED BY
or LINES TERMINATED
BY
followed by the FIELDS TERMINATED
BY
value causes LOAD
DATA
to stop reading a field or line too early.
This happens because LOAD
DATA
cannot properly determine where the field or
line value ends.
The following example loads all columns of the
persondata
table:
LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata;
By default, when no column list is provided at the end of the
LOAD DATA
statement, input lines
are expected to contain a field for each table column. If you
want to load only some of a table's columns, specify a column
list:
LOAD DATA INFILE 'persondata.txt' INTO TABLE persondata (col_name_or_user_var
[,col_name_or_user_var
] ...);
You must also specify a column list if the order of the fields in the input file differs from the order of the columns in the table. Otherwise, MySQL cannot tell how to match input fields with table columns.
Each col_name_or_user_var
value is
either a column name or a user variable. With user variables,
the SET
clause enables you to perform
preprocessing transformations on their values before assigning
the result to columns.
User variables in the SET
clause can be used
in several ways. The following example uses the first input
column directly for the value of t1.column1
,
and assigns the second input column to a user variable that is
subjected to a division operation before being used for the
value of t1.column2
:
LOAD DATA INFILE 'file.txt' INTO TABLE t1 (column1, @var1) SET column2 = @var1/100;
The SET
clause can be used to supply values
not derived from the input file. The following statement sets
column3
to the current date and time:
LOAD DATA INFILE 'file.txt' INTO TABLE t1 (column1, column2) SET column3 = CURRENT_TIMESTAMP;
You can also discard an input value by assigning it to a user variable and not assigning the variable to a table column:
LOAD DATA INFILE 'file.txt' INTO TABLE t1 (column1, @dummy, column2, @dummy, column3);
Use of the column/variable list and SET
clause is subject to the following restrictions:
Assignments in the SET
clause should have
only column names on the left hand side of assignment
operators.
You can use subqueries in the right hand side of
SET
assignments. A subquery that returns
a value to be assigned to a column may be a scalar subquery
only. Also, you cannot use a subquery to select from the
table that is being loaded.
Lines ignored by an IGNORE
clause are not
processed for the column/variable list or
SET
clause.
User variables cannot be used when loading data with fixed-row format because user variables do not have a display width.
When processing an input line, LOAD
DATA
splits it into fields and uses the values
according to the column/variable list and the
SET
clause, if they are present. Then the
resulting row is inserted into the table. If there are
BEFORE INSERT
or AFTER
INSERT
triggers for the table, they are activated
before or after inserting the row, respectively.
If an input line has too many fields, the extra fields are ignored and the number of warnings is incremented.
If an input line has too few fields, the table columns for which input fields are missing are set to their default values. Default value assignment is described in Section 11.7, “Data Type Default Values”.
An empty field value is interpreted different from a missing field:
For string types, the column is set to the empty string.
For numeric types, the column is set to
0
.
For date and time types, the column is set to the appropriate “zero” value for the type. See Section 11.3, “Date and Time Types”.
These are the same values that result if you assign an empty
string explicitly to a string, numeric, or date or time type
explicitly in an INSERT
or
UPDATE
statement.
Treatment of empty or incorrect field values differs from that
just described if the SQL mode is set to a restrictive value.
For example, if sql_mode
is set
to TRADITIONAL
, conversion of
an empty value or a value such as 'x'
for a
numeric column results in an error, not conversion to 0. (With
LOCAL
or IGNORE
, warnings
occur rather than errors, even with a restrictive
sql_mode
value, and the row is
inserted using the same closest-value behavior used for
nonrestrictive SQL modes. This occurs because the server has no
way to stop transmission of the file in the middle of the
operation.)
TIMESTAMP
columns are set to the
current date and time only if there is a NULL
value for the column (that is, \N
) and the
column is not declared to permit NULL
values,
or if the TIMESTAMP
column's
default value is the current timestamp and it is omitted from
the field list when a field list is specified.
LOAD DATA
regards all input as
strings, so you cannot use numeric values for
ENUM
or
SET
columns the way you can with
INSERT
statements. All
ENUM
and
SET
values must be specified as
strings.
BIT
values cannot be loaded
directly using binary notation (for example,
b'011010'
). To work around this, use the
SET
clause to strip off the leading
b'
and trailing '
and
perform a base-2 to base-10 conversion so that MySQL loads the
values into the BIT
column
properly:
shell>cat /tmp/bit_test.txt
b'10' b'1111111' shell>mysql test
mysql>LOAD DATA INFILE '/tmp/bit_test.txt'
INTO TABLE bit_test (@var1)
SET b = CAST(CONV(MID(@var1, 3, LENGTH(@var1)-3), 2, 10) AS UNSIGNED);
Query OK, 2 rows affected (0.00 sec) Records: 2 Deleted: 0 Skipped: 0 Warnings: 0 mysql>SELECT BIN(b+0) FROM bit_test;
+----------+ | BIN(b+0) | +----------+ | 10 | | 1111111 | +----------+ 2 rows in set (0.00 sec)
For BIT
values in
0b
binary notation (for example,
0b011010
), use this SET
clause instead to strip off the leading 0b
:
SET b = CAST(CONV(MID(@var1, 3, LENGTH(@var1)-2), 2, 10) AS UNSIGNED)
When the LOAD DATA
statement
finishes, it returns an information string in the following
format:
Records: 1 Deleted: 0 Skipped: 0 Warnings: 0
Warnings occur under the same circumstances as when values are
inserted using the INSERT
statement (see Section 13.2.6, “INSERT Syntax”), except that
LOAD DATA
also generates warnings
when there are too few or too many fields in the input row.
You can use SHOW WARNINGS
to get
a list of the first
max_error_count
warnings as
information about what went wrong. See
Section 13.7.6.40, “SHOW WARNINGS Syntax”.
If you are using the C API, you can get information about the
statement by calling the
mysql_info()
function. See
Section 28.7.7.36, “mysql_info()”.
On Unix, if you need LOAD DATA
to
read from a pipe, you can use the following technique (the
example loads a listing of the /
directory
into the table db1.t1
):
mkfifo /mysql/data/db1/ls.dat chmod 666 /mysql/data/db1/ls.dat find / -ls > /mysql/data/db1/ls.dat & mysql -e "LOAD DATA INFILE 'ls.dat' INTO TABLE t1" db1
Here you must run the command that generates the data to be loaded and the mysql commands either on separate terminals, or run the data generation process in the background (as shown in the preceding example). If you do not do this, the pipe will block until data is read by the mysql process.
LOAD XML [LOW_PRIORITY | CONCURRENT] [LOCAL] INFILE 'file_name
' [REPLACE | IGNORE] INTO TABLE [db_name
.]tbl_name
[CHARACTER SETcharset_name
] [ROWS IDENTIFIED BY '<tagname
>'] [IGNOREnumber
{LINES | ROWS}] [(field_name_or_user_var
[,field_name_or_user_var
] ...)] [SETcol_name
={expr
| DEFAULT}, [,col_name
={expr
| DEFAULT}] ...]
The LOAD XML
statement reads data
from an XML file into a table. The
file_name
must be given as a literal
string. The tagname
in the optional
ROWS IDENTIFIED BY
clause must also be given as
a literal string, and must be surrounded by angle brackets
(<
and >
).
LOAD XML
acts as the complement of
running the mysql client in XML output mode
(that is, starting the client with the
--xml
option). To write data from a
table to an XML file, you can invoke the mysql
client with the --xml
and
-e
options from
the system shell, as shown here:
shell> mysql --xml -e 'SELECT * FROM mydb.mytable' > file.xml
To read the file back into a table, use LOAD
XML
. By default, the <row>
element is considered to be the equivalent of a database table
row; this can be changed using the ROWS IDENTIFIED
BY
clause.
This statement supports three different XML formats:
Column names as attributes and column values as attribute values:
<row
column1
="value1
"column2
="value2
" .../>
Column names as tags and column values as the content of these tags:
<row
> <column1
>value1
</column1
> <column2
>value2
</column2
> </row
>
Column names are the name
attributes of
<field>
tags, and values are the
contents of these tags:
<row> <field name='column1
'>value1
</field> <field name='column2
'>value2
</field> </row>
This is the format used by other MySQL tools, such as mysqldump.
All three formats can be used in the same XML file; the import routine automatically detects the format for each row and interprets it correctly. Tags are matched based on the tag or attribute name and the column name.
The following clauses work essentially the same way for
LOAD XML
as they do for
LOAD DATA
:
LOW_PRIORITY
or
CONCURRENT
LOCAL
REPLACE
or IGNORE
CHARACTER SET
SET
See Section 13.2.7, “LOAD DATA Syntax”, for more information about these clauses.
(
is a list of one or more comma-separated XML fields
or user variables. The name of a user variable used for this
purpose must match the name of a field from the XML file, prefixed
with field_name_or_user_var
,
...)@
. You can use field names to select only
desired fields. User variables can be employed to store the
corresponding field values for subsequent re-use.
The IGNORE
or number
LINESIGNORE
clause causes the
first number
ROWSnumber
rows in the XML file to be
skipped. It is analogous to the LOAD
DATA
statement's IGNORE ... LINES
clause.
Suppose that we have a table named person
,
created as shown here:
USE test; CREATE TABLE person ( person_id INT NOT NULL PRIMARY KEY, fname VARCHAR(40) NULL, lname VARCHAR(40) NULL, created TIMESTAMP );
Suppose further that this table is initially empty.
Now suppose that we have a simple XML file
person.xml
, whose contents are as shown here:
<list> <person person_id="1" fname="Kapek" lname="Sainnouine"/> <person person_id="2" fname="Sajon" lname="Rondela"/> <person person_id="3"><fname>Likame</fname><lname>Örrtmons</lname></person> <person person_id="4"><fname>Slar</fname><lname>Manlanth</lname></person> <person><field name="person_id">5</field><field name="fname">Stoma</field> <field name="lname">Milu</field></person> <person><field name="person_id">6</field><field name="fname">Nirtam</field> <field name="lname">Sklöd</field></person> <person person_id="7"><fname>Sungam</fname><lname>Dulbåd</lname></person> <person person_id="8" fname="Sraref" lname="Encmelt"/> </list>
Each of the permissible XML formats discussed previously is represented in this example file.
To import the data in person.xml
into the
person
table, you can use this statement:
mysql>LOAD XML LOCAL INFILE 'person.xml'
->INTO TABLE person
->ROWS IDENTIFIED BY '<person>';
Query OK, 8 rows affected (0.00 sec) Records: 8 Deleted: 0 Skipped: 0 Warnings: 0
Here, we assume that person.xml
is located in
the MySQL data directory. If the file cannot be found, the
following error results:
ERROR 2 (HY000): File '/person.xml' not found (Errcode: 2)
The ROWS IDENTIFIED BY '<person>'
clause
means that each <person>
element in the
XML file is considered equivalent to a row in the table into which
the data is to be imported. In this case, this is the
person
table in the test
database.
As can be seen by the response from the server, 8 rows were
imported into the test.person
table. This can
be verified by a simple SELECT
statement:
mysql> SELECT * FROM person;
+-----------+--------+------------+---------------------+
| person_id | fname | lname | created |
+-----------+--------+------------+---------------------+
| 1 | Kapek | Sainnouine | 2007-07-13 16:18:47 |
| 2 | Sajon | Rondela | 2007-07-13 16:18:47 |
| 3 | Likame | Örrtmons | 2007-07-13 16:18:47 |
| 4 | Slar | Manlanth | 2007-07-13 16:18:47 |
| 5 | Stoma | Nilu | 2007-07-13 16:18:47 |
| 6 | Nirtam | Sklöd | 2007-07-13 16:18:47 |
| 7 | Sungam | Dulbåd | 2007-07-13 16:18:47 |
| 8 | Sreraf | Encmelt | 2007-07-13 16:18:47 |
+-----------+--------+------------+---------------------+
8 rows in set (0.00 sec)
This shows, as stated earlier in this section, that any or all of
the 3 permitted XML formats may appear in a single file and be
read using LOAD XML
.
The inverse of the import operation just shown—that is, dumping MySQL table data into an XML file—can be accomplished using the mysql client from the system shell, as shown here:
shell>mysql --xml -e "SELECT * FROM test.person" > person-dump.xml
shell>cat person-dump.xml
<?xml version="1.0"?> <resultset statement="SELECT * FROM test.person" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"> <row> <field name="person_id">1</field> <field name="fname">Kapek</field> <field name="lname">Sainnouine</field> </row> <row> <field name="person_id">2</field> <field name="fname">Sajon</field> <field name="lname">Rondela</field> </row> <row> <field name="person_id">3</field> <field name="fname">Likema</field> <field name="lname">Örrtmons</field> </row> <row> <field name="person_id">4</field> <field name="fname">Slar</field> <field name="lname">Manlanth</field> </row> <row> <field name="person_id">5</field> <field name="fname">Stoma</field> <field name="lname">Nilu</field> </row> <row> <field name="person_id">6</field> <field name="fname">Nirtam</field> <field name="lname">Sklöd</field> </row> <row> <field name="person_id">7</field> <field name="fname">Sungam</field> <field name="lname">Dulbåd</field> </row> <row> <field name="person_id">8</field> <field name="fname">Sreraf</field> <field name="lname">Encmelt</field> </row> </resultset>
The --xml
option causes the
mysql client to use XML formatting for its
output; the -e
option causes the client to execute the SQL statement
immediately following the option. See Section 4.5.1, “mysql — The MySQL Command-Line Client”.
You can verify that the dump is valid by creating a copy of the
person
table and importing the dump file into
the new table, like this:
mysql>USE test;
mysql>CREATE TABLE person2 LIKE person;
Query OK, 0 rows affected (0.00 sec) mysql>LOAD XML LOCAL INFILE 'person-dump.xml'
->INTO TABLE person2;
Query OK, 8 rows affected (0.01 sec) Records: 8 Deleted: 0 Skipped: 0 Warnings: 0 mysql>SELECT * FROM person2;
+-----------+--------+------------+---------------------+ | person_id | fname | lname | created | +-----------+--------+------------+---------------------+ | 1 | Kapek | Sainnouine | 2007-07-13 16:18:47 | | 2 | Sajon | Rondela | 2007-07-13 16:18:47 | | 3 | Likema | Örrtmons | 2007-07-13 16:18:47 | | 4 | Slar | Manlanth | 2007-07-13 16:18:47 | | 5 | Stoma | Nilu | 2007-07-13 16:18:47 | | 6 | Nirtam | Sklöd | 2007-07-13 16:18:47 | | 7 | Sungam | Dulbåd | 2007-07-13 16:18:47 | | 8 | Sreraf | Encmelt | 2007-07-13 16:18:47 | +-----------+--------+------------+---------------------+ 8 rows in set (0.00 sec)
There is no requirement that every field in the XML file be
matched with a column in the corresponding table. Fields which
have no corresponding columns are skipped. You can see this by
first emptying the person2
table and dropping
the created
column, then using the same
LOAD XML
statement we just employed
previously, like this:
mysql>TRUNCATE person2;
Query OK, 8 rows affected (0.26 sec) mysql>ALTER TABLE person2 DROP COLUMN created;
Query OK, 0 rows affected (0.52 sec) Records: 0 Duplicates: 0 Warnings: 0 mysql>SHOW CREATE TABLE person2\G
*************************** 1. row *************************** Table: person2 Create Table: CREATE TABLE `person2` ( `person_id` int(11) NOT NULL, `fname` varchar(40) DEFAULT NULL, `lname` varchar(40) DEFAULT NULL, PRIMARY KEY (`person_id`) ) ENGINE=InnoDB DEFAULT CHARSET=utf8 1 row in set (0.00 sec) mysql>LOAD XML LOCAL INFILE 'person-dump.xml'
->INTO TABLE person2;
Query OK, 8 rows affected (0.01 sec) Records: 8 Deleted: 0 Skipped: 0 Warnings: 0 mysql>SELECT * FROM person2;
+-----------+--------+------------+ | person_id | fname | lname | +-----------+--------+------------+ | 1 | Kapek | Sainnouine | | 2 | Sajon | Rondela | | 3 | Likema | Örrtmons |