SQL in a Nutshell, 2nd Edition

ANSI SQL Window Functions

SQL2003 allows for a window_clause in aggregate function calls, the addition of which makes those functions into window functions. Both Oracle and DB2 support this window function syntax. This section describes how to use the window_clause within Oracle and DB2.

Tip

Oracle tends to refer to window functions as analytic functions.

Window, or analytic, functions are similar to standard aggregate functions in that they operate on multiple rows, or groups of rows, within the result set returned from a query. However, the groups of rows that a window function operates on are defined not by a GROUP BY clause, but by partitioning and windowing clauses. Furthermore, the order within these groups is defined by an ordering clause, but that order only affects function evaluation, and has no effect on the order in which rows are returned by the query.

Tip

Window functions are the last items in a query to be evaluated except for the ORDER BY clause. Because of this late evaluation, window functions cannot be used within the WHERE, GROUP BY, or HAVING clauses.