This page is a quick reference checkpoint for COUNT OVER in IBM DB2: behavior, syntax rules, edge cases, and a minimal example; plus the official vendor documentation.
COUNT OVER returns the number of rows in the window frame.
COUNT() OVER(...) returns the count of non-NULL values (or rows with COUNT() ) in the partition or window frame; if DISTINCT is used, OVER cannot include ORDER BY or frame clause. NULL values are ignored unless all values are NULL, in which case result is NULL. Aggregation specification cannot be used in WHERE, GROUP BY, HAVING, VALUES, SET, or as an argument to another aggregate.
If this behavior feels unintuitive, the tutorial below explains the underlying pattern step-by-step.
COUNT() can be used with an OVER() clause as part of the OLAP aggregation-specification
SELECT COUNT(*) OVER (PARTITION BY department_id) FROM employee;
If you came here to confirm syntax, you’re done. If you came here to get better at window functions, choose your next step.
COUNT OVER is part of a bigger window-function pattern. If you want the “why”, start here: Aggregate Window Functions
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 IBM DB2 Documentation →Looking for more functions across all SQL dialects? Visit the full SQL Dialects & Window Functions Documentation.