LAG accesses data from a previous row in the same result set without the use of a self-join in SQL Server 2016. LAG provides access to a row at a given physical offset that comes before the current row. Usethis analytic function in a SELECT statement to compare values in the current row with values in a previous row.
Use ORDER BY Year, not ORDER BY Territory.
Example: The following example uses the LAG function to return the difference in sales quotas fora 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');