This page is a quick reference checkpoint for LAG in SQL Server: behavior, syntax rules, edge cases, and a minimal example; plus the official vendor documentation.
LAG lets you look back at a previous row's value within the same partition.
LAG accesses a row at a specified physical offset that comes before the current row in the same result set; if offset is beyond partition returns default (or NULL); IGNORE NULLS / RESPECT NULLS supported from SQL Server 2022 (16.x).
If this behavior feels unintuitive, the tutorial below explains the underlying pattern step-by-step.
LAG(scalar_expression [ , offset ] [ , default ]) [ IGNORE NULLS | RESPECT NULLS ] OVER ( [ partition_by_clause ] order_by_clause )
SELECT LAG(SalesQuota,1,0) OVER (ORDER BY QuotaDate) AS PreviousQuota FROM Sales.SalesPersonQuotaHistory;
If you came here to confirm syntax, you’re done. If you came here to get better at window functions, choose your next step.
LAG is part of a bigger window-function pattern. If you want the “why”, start here: Lead Lag
Reading docs is useful. Writing the query correctly under pressure is the skill.
For the authoritative spec, use the vendor docs. This page is the fast “sanity check”.
View SQL Server Documentation →Looking for more functions across all SQL dialects? Visit the full SQL Dialects & Window Functions Documentation.