This page is a quick reference checkpoint for LAG in Postgres: 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.
Returns the value from a row that precedes the current row within the partition; if the offset row does not exist, NULL or the provided default is returned.
If this behavior feels unintuitive, the tutorial below explains the underlying pattern step-by-step.
LAG(value [, offset [, default ]]) is listed as a window function and requires an OVER clause.
SELECT LAG(salary, 1) OVER (PARTITION BY dept ORDER BY salary) FROM empsalary;
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 Postgres Documentation →Looking for more functions across all SQL dialects? Visit the full SQL Dialects & Window Functions Documentation.