In SQL, a window function or analytic function is a function which uses values from one or multiple rows to return a value for each row. (This contrasts with an aggregate function, which returns a single value for multiple rows.) Window functions have an OVER clause; any function without an OVER clause is not a window function, but rather an aggregate or single-row (scalar) function.

As an example, here is a query which uses a window function to compare the salary of each employee with the average salary of their department (example from the PostgreSQL documentation):

Output:

The PARTITION BY clause groups rows into partitions, and the function is applied to each partition separately. If the PARTITION BY clause is omitted (such as with an empty OVER() clause), then the entire result set is treated as a single partition. For this query, the average salary reported would be the average taken over all rows.

Window functions are evaluated after aggregation (after the GROUP BY clause and non-window aggregate functions, for example).

According to the PostgreSQL documentation, a window function has the syntax of one of the following:

where window_definition has syntax:

frame_clause has the syntax of one of the following: