Transact-SQL Syntax ConventionsCombines the results of two or more queries into a single result set that includes all the rows that belong to all queries in the union. The UNION operation is different from using joins that combine columns from two tables.
The following are basic rules for combining the result sets of two queries by using UNION:
The number and the order of the columns must be the same in all queries.
The data types must be compatible.
Transact-SQL Syntax Conventions
{ <query_specification> | ( <query_expression> ) }
UNION [ ALL ]
<query_specification | ( <query_expression> )
[ UNION [ ALL ] <query_specification> | ( <query_expression> )
[ ...n ] ] <query_specification> | ( <query_expression> ) Is a query specification or query expression that returns data to be combined with the data from another query specification or query expression. The definitions of the columns that are part of a UNION operation do not have to be the same, but they must be compatible through implicit conversion. When data types differ, the resulting data type is determined based on the rules for data type precedence. When the types are the same but differ in precision, scale, or length, the result is determined based on the same rules for combining expressions. For more information, see Precision, Scale, and Length (Transact-SQL).
Columns of the xml data type must be equivalent. All columns must be either typed to an XML schema or untyped. If typed, they must be typed to the same XML schema collection.
UNION
Specifies that multiple result sets are to be combined and returned as a single result set.
ALL
Incorporates all rows into the results. This includes duplicates. If not specified, duplicate rows are removed.
In the following example, the result set includes the contents of the ProductModelID and Name columns of both the ProductModel and Gloves tables.
-- Uses AdventureWorks
IF OBJECT_ID ('dbo.Gloves', 'U') IS NOT NULL
DROP TABLE dbo.Gloves;
GO
-- Create Gloves table.
SELECT ProductModelID, Name
INTO dbo.Gloves
FROM Production.ProductModel
WHERE ProductModelID IN (3, 4);
GO
-- Here is the simple union.
-- Uses AdventureWorks
SELECT ProductModelID, Name
FROM Production.ProductModel
WHERE ProductModelID NOT IN (3, 4)
UNION
SELECT ProductModelID, Name
FROM dbo.Gloves
ORDER BY Name;
GO In the following example, the INTO clause in the second SELECT statement specifies that the table named ProductResults holds the final result set of the union of the designated columns of the ProductModel and Gloves tables. Note that the Gloves table is created in the first SELECT statement.
-- Uses AdventureWorks
IF OBJECT_ID ('dbo.ProductResults', 'U') IS NOT NULL
DROP TABLE dbo.ProductResults;
GO
IF OBJECT_ID ('dbo.Gloves', 'U') IS NOT NULL
DROP TABLE dbo.Gloves;
GO
-- Create Gloves table.
SELECT ProductModelID, Name
INTO dbo.Gloves
FROM Production.ProductModel
WHERE ProductModelID IN (3, 4);
GO
-- Uses AdventureWorks
SELECT ProductModelID, Name
INTO dbo.ProductResults
FROM Production.ProductModel
WHERE ProductModelID NOT IN (3, 4)
UNION
SELECT ProductModelID, Name
FROM dbo.Gloves;
GO
SELECT ProductModelID, Name
FROM dbo.ProductResults;
The order of certain parameters used with the UNION clause is important. The following example shows the incorrect and correct use of UNION in two SELECT statements in which a column is to be renamed in the output.
-- Uses AdventureWorks
IF OBJECT_ID ('dbo.Gloves', 'U') IS NOT NULL
DROP TABLE dbo.Gloves;
GO
-- Create Gloves table.
SELECT ProductModelID, Name
INTO dbo.Gloves
FROM Production.ProductModel
WHERE ProductModelID IN (3, 4);
GO
/* INCORRECT */
-- Uses AdventureWorks
SELECT ProductModelID, Name
FROM Production.ProductModel
WHERE ProductModelID NOT IN (3, 4)
ORDER BY Name
UNION
SELECT ProductModelID, Name
FROM dbo.Gloves;
GO
/* CORRECT */
-- Uses AdventureWorks
SELECT ProductModelID, Name
FROM Production.ProductModel
WHERE ProductModelID NOT IN (3, 4)
UNION
SELECT ProductModelID, Name
FROM dbo.Gloves
ORDER BY Name;
GO
The following examples use UNION to combine the results of three tables that all have the same 5 rows of data. The first example uses UNION ALL to show the duplicated records, and returns all 15 rows. The second example uses UNION without ALL to eliminate the duplicate rows from the combined results of the three SELECT statements, and returns 5 rows.
The third example uses ALL with the first UNION and parentheses enclose the second UNION that is not using ALL. The second UNION is processed first because it is in parentheses, and returns 5 rows because the ALL option is not used and the duplicates are removed. These 5 rows are combined with the results of the first SELECT by using the UNION ALL keywords. This does not remove the duplicates between the two sets of 5 rows. The final result has 10 rows.
-- Uses AdventureWorks
IF OBJECT_ID ('dbo.EmployeeOne', 'U') IS NOT NULL
DROP TABLE dbo.EmployeeOne;
GO
IF OBJECT_ID ('dbo.EmployeeTwo', 'U') IS NOT NULL
DROP TABLE dbo.EmployeeTwo;
GO
IF OBJECT_ID ('dbo.EmployeeThree', 'U') IS NOT NULL
DROP TABLE dbo.EmployeeThree;
GO
SELECT pp.LastName, pp.FirstName, e.JobTitle
INTO dbo.EmployeeOne
FROM Person.Person AS pp JOIN HumanResources.Employee AS e
ON e.BusinessEntityID = pp.BusinessEntityID
WHERE LastName = 'Johnson';
GO
SELECT pp.LastName, pp.FirstName, e.JobTitle
INTO dbo.EmployeeTwo
FROM Person.Person AS pp JOIN HumanResources.Employee AS e
ON e.BusinessEntityID = pp.BusinessEntityID
WHERE LastName = 'Johnson';
GO
SELECT pp.LastName, pp.FirstName, e.JobTitle
INTO dbo.EmployeeThree
FROM Person.Person AS pp JOIN HumanResources.Employee AS e
ON e.BusinessEntityID = pp.BusinessEntityID
WHERE LastName = 'Johnson';
GO
-- Union ALL
SELECT LastName, FirstName, JobTitle
FROM dbo.EmployeeOne
UNION ALL
SELECT LastName, FirstName ,JobTitle
FROM dbo.EmployeeTwo
UNION ALL
SELECT LastName, FirstName,JobTitle
FROM dbo.EmployeeThree;
GO
SELECT LastName, FirstName,JobTitle
FROM dbo.EmployeeOne
UNION
SELECT LastName, FirstName, JobTitle
FROM dbo.EmployeeTwo
UNION
SELECT LastName, FirstName, JobTitle
FROM dbo.EmployeeThree;
GO
SELECT LastName, FirstName,JobTitle
FROM dbo.EmployeeOne
UNION ALL
(
SELECT LastName, FirstName, JobTitle
FROM dbo.EmployeeTwo
UNION
SELECT LastName, FirstName, JobTitle
FROM dbo.EmployeeThree
);
GO
In the following example, the result set includes the contents of the CustomerKey columns of both the FactInternetSales and DimCustomer tables. Since the ALL keyword is not used, duplicates are excluded from the results.
-- Uses AdventureWorks
SELECT CustomerKey
FROM FactInternetSales
UNION
SELECT CustomerKey
FROM DimCustomer
ORDER BY CustomerKey; When any SELECT statement in a UNION statement includes an ORDER BY clause, that clause should be placed after all SELECT statements. The following example shows the incorrect and correct use of UNION in two SELECT statements in which a column is ordered with ORDER BY.
-- Uses AdventureWorks
-- INCORRECT
SELECT CustomerKey
FROM FactInternetSales
ORDER BY CustomerKey
UNION
SELECT CustomerKey
FROM DimCustomer
ORDER BY CustomerKey;
-- CORRECT
USE AdventureWorksPDW2012;
SELECT CustomerKey
FROM FactInternetSales
UNION
SELECT CustomerKey
FROM DimCustomer
ORDER BY CustomerKey; The following example shows the incorrect and correct use of UNION in two SELECT statements where WHERE and ORDER BY are needed.
-- Uses AdventureWorks
-- INCORRECT
SELECT CustomerKey
FROM FactInternetSales
WHERE CustomerKey >= 11000
ORDER BY CustomerKey
UNION
SELECT CustomerKey
FROM DimCustomer
ORDER BY CustomerKey;
-- CORRECT
USE AdventureWorksPDW2012;
SELECT CustomerKey
FROM FactInternetSales
WHERE CustomerKey >= 11000
UNION
SELECT CustomerKey
FROM DimCustomer
ORDER BY CustomerKey; The following examples use UNION to combine the results of the same table in order to demonstrate the effects of ALL and parentheses when using UNION.
The first example uses UNION ALL to show duplicated records and returns each row in the source table three times. The second example uses UNION without ALL to eliminate the duplicate rows from the combined results of the three SELECT statements and returns only the unduplicated rows from the source table.
The third example uses ALL with the first UNION and parentheses enclosing the second UNION that is not using ALL. The second UNION is processed first because it is in parentheses. It returns only the unduplicated rows from the table because the ALL option is not used and duplicates are removed. These rows are combined with the results of the first SELECT by using the UNION ALL keywords. This does not remove the duplicates between the two sets.
-- Uses AdventureWorks
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer
UNION ALL
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer
UNION ALL
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer;
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer
UNION
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer
UNION
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer;
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer
UNION ALL
(
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer
UNION
SELECT CustomerKey, FirstName, LastName
FROM DimCustomer
);