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LAG (Transact-SQL)

Accesses data from a previous row in the same result set without the use of a self-join in SQL Server 2014. 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.

Applies to: SQL Server (SQL Server 2012 through current version).

Topic link icon 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.

A. Compare values between years

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

B. Compare values within partitions

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

C. Specifying arbitrary expressions

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

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