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LEAD (Transact-SQL)

Accesses data from a subsequent row in the same result set without the use of a self-join in SQL Server 2014. LEAD provides access to a row at a given physical offset that follows the current row. Use this analytic function in a SELECT statement to compare values in the current row with values in a following row.

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

Topic link icon Transact-SQL Syntax Conventions (Transact-SQL)

LEAD ( 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 forward 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. When partition_by_clause is specified, it determines the order of the data in each 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.

LEAD is nondeterministic. For more information, see Deterministic and Nondeterministic Functions.

A. Compare values between years

The query uses the LEAD function to return the difference in sales quotas for a specific employee over subsequent years. Notice that because there is no lead value available for the last row, the default of zero (0) is returned.

USE AdventureWorks2012;
GO
SELECT BusinessEntityID, YEAR(QuotaDate) AS SalesYear, SalesQuota AS CurrentQuota, 
    LEAD(SalesQuota, 1,0) OVER (ORDER BY YEAR(QuotaDate)) AS NextQuota
FROM Sales.SalesPersonQuotaHistory
WHERE BusinessEntityID = 275 and YEAR(QuotaDate) IN ('2005','2006');

Here is the result set.

BusinessEntityID SalesYear   CurrentQuota          NextQuota
---------------- ----------- --------------------- ---------------------
275              2005        367000.00             556000.00
275              2005        556000.00             502000.00
275              2006        502000.00             550000.00
275              2006        550000.00             1429000.00
275              2006        1429000.00            1324000.00
275              2006        1324000.00            0.00

B. Compare values within partitions

The following example uses the LEAD function to compare year-to-date sales between employees. The PARTITION BY clause is specified to partition the rows in the result set by sales territory. The LEAD function is applied to each partition separately and computation restarts for each partition. The ORDER BY clause specified in the OVER clause orders the rows in each partition before the function is applied. The ORDER BY clause in the SELECT statement orders the rows in the whole result set. Notice that because there is no lead value available for the last row of each partition, the default of zero (0) is returned.

USE AdventureWorks2012;
GO
SELECT TerritoryName, BusinessEntityID, SalesYTD, 
       LEAD (SalesYTD, 1, 0) OVER (PARTITION BY TerritoryName ORDER BY SalesYTD DESC) AS NextRepSales
FROM Sales.vSalesPerson
WHERE TerritoryName IN (N'Northwest', N'Canada') 
ORDER BY TerritoryName;

Here is the result set.

TerritoryName            BusinessEntityID SalesYTD              NextRepSales
-----------------------  ---------------- --------------------- ---------------------
Canada                   282              2604540.7172          1453719.4653
Canada                   278              1453719.4653          0.00
Northwest                284              1576562.1966          1573012.9383
Northwest                283              1573012.9383          1352577.1325
Northwest                280              1352577.1325          0.00

C. Specifying arbitrary expressions

The following example demonstrates specifying a variety of arbitrary expressions in the LEAD 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, 
    LEAD(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          8
2           4           2
1           NULL        2
3           1           0
2           NULL        NULL
1           5           -2

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