Transact-SQL Syntax Conventions (Transact-SQL)Accesses data from a previous row in the same result set without the use of a self-join starting with SQL Server 2012 (11.x) LAG provides access to a row at a given physical offset that comes before the current row. Use this analytic function in a SELECT statement to compare values in the current row with values in a previous row.
Transact-SQL Syntax Conventions (Transact-SQL)
LAG (scalar_expression [,offset] [,default])
OVER ( [ partition_by_clause ] order_by_clause ) scalar_expression
The value to be returned based on the specified offset. It is an expression of any type that returns a single (scalar) value. scalar_expression cannot be an analytic function.
offset
The number of rows back from the current row from which to obtain a value. If not specified, the default is 1. offset can be a column, subquery, or other expression that evaluates to a positive integer or can be implicitly converted to bigint. offset cannot be a negative value or an analytic function.
default
The value to return when scalar_expression at offset is NULL. If a default value is not specified, NULL is returned. default can be a column, subquery, or other expression, but it cannot be an analytic function. default must be type-compatible with scalar_expression.
OVER ( [ partition_by_clause ] order_by_clause)
partition_by_clause divides the result set produced by the FROM clause into partitions to which the function is applied. If not specified, the function treats all rows of the query result set as a single group. order_by_clause determines the order of the data before the function is applied. If partition_by_clause is specified, it determines the order of the data in the partition. The order_by_clause is required. For more information, see OVER Clause (Transact-SQL).
The data type of the specified scalar_expression. NULL is returned if scalar_expression is nullable or default is set to NULL.
LAG is nondeterministic. For more information, see Deterministic and Nondeterministic Functions.
The following example uses the LAG function to return the difference in sales quotas for a specific employee over previous years. Notice that because there is no lag value available for the first row, the default of zero (0) is returned.
USE AdventureWorks2012;
GO
SELECT BusinessEntityID, YEAR(QuotaDate) AS SalesYear, SalesQuota AS CurrentQuota,
LAG(SalesQuota, 1,0) OVER (ORDER BY YEAR(QuotaDate)) AS PreviousQuota
FROM Sales.SalesPersonQuotaHistory
WHERE BusinessEntityID = 275 and YEAR(QuotaDate) IN ('2005','2006'); Here is the result set.
BusinessEntityID SalesYear CurrentQuota PreviousQuota
---------------- ----------- --------------------- ---------------------
275 2005 367000.00 0.00
275 2005 556000.00 367000.00
275 2006 502000.00 556000.00
275 2006 550000.00 502000.00
275 2006 1429000.00 550000.00
275 2006 1324000.00 1429000.00
The following example uses the LAG function to compare year-to-date sales between employees. The PARTITION BY clause is specified to divide the rows in the result set by sales territory. The LAG function is applied to each partition separately and computation restarts for each partition. The ORDER BY clause in the OVER clause orders the rows in each partition. The ORDER BY clause in the SELECT statement sorts the rows in the whole result set. Notice that because there is no lag value available for the first row of each partition, the default of zero (0) is returned.
USE AdventureWorks2012;
GO
SELECT TerritoryName, BusinessEntityID, SalesYTD,
LAG (SalesYTD, 1, 0) OVER (PARTITION BY TerritoryName ORDER BY SalesYTD DESC) AS PrevRepSales
FROM Sales.vSalesPerson
WHERE TerritoryName IN (N'Northwest', N'Canada')
ORDER BY TerritoryName; Here is the result set.
TerritoryName BusinessEntityID SalesYTD PrevRepSales
----------------------- ---------------- --------------------- ---------------------
Canada 282 2604540.7172 0.00
Canada 278 1453719.4653 2604540.7172
Northwest 284 1576562.1966 0.00
Northwest 283 1573012.9383 1576562.1966
Northwest 280 1352577.1325 1573012.9383
The following example demonstrates specifying a variety of arbitrary expressions in the LAG function syntax.
CREATE TABLE T (a int, b int, c int);
GO
INSERT INTO T VALUES (1, 1, -3), (2, 2, 4), (3, 1, NULL), (4, 3, 1), (5, 2, NULL), (6, 1, 5);
SELECT b, c,
LAG(2*c, b*(SELECT MIN(b) FROM T), -c/2.0) OVER (ORDER BY a) AS i
FROM T; Here is the result set.
b c i
----------- ----------- -----------
1 -3 1
2 4 -2
1 NULL 8
3 1 -6
2 NULL NULL
1 5 NULL The following example demonstrates the LAG function. The query uses the LAG function to return the difference in sales quotas for a specific employee over previous calendar quarters. Notice that because there is no lag value available for the first row, the default of zero (0) is returned.
-- Uses AdventureWorks
SELECT CalendarYear, CalendarQuarter, SalesAmountQuota AS SalesQuota,
LAG(SalesAmountQuota,1,0) OVER (ORDER BY CalendarYear, CalendarQuarter) AS PrevQuota,
SalesAmountQuota - LAG(SalesAmountQuota,1,0) OVER (ORDER BY CalendarYear, CalendarQuarter) AS Diff
FROM dbo.FactSalesQuota
WHERE EmployeeKey = 272 AND CalendarYear IN (2001, 2002)
ORDER BY CalendarYear, CalendarQuarter; Here is the result set.
Year Quarter SalesQuota PrevQuota Diff
---- ------- ---------- --------- -------------
2001 3 28000.0000 0.0000 28000.0000
2001 4 7000.0000 28000.0000 -21000.0000
2001 1 91000.0000 7000.0000 84000.0000
2002 2 140000.0000 91000.0000 49000.0000
2002 3 7000.0000 140000.0000 -70000.0000
2002 4 154000.0000 7000.0000 84000.0000