This page is a quick reference checkpoint for COUNT OVER in MySQL: 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.
As a window function, COUNT() returns a value for each row in the window rather than collapsing rows; COUNT() counts all rows, COUNT(expr) counts non-NULL values.
If this behavior feels unintuitive, the tutorial below explains the underlying pattern step-by-step.
COUNT(expr) can be used as a window function with an OVER clause that may include PARTITION BY and ORDER BY.
SELECT COUNT() OVER (PARTITION BY dept ORDER BY salary) AS dept_count FROM employees;
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 MySQL Documentation →Looking for more functions across all SQL dialects? Visit the full SQL Dialects & Window Functions Documentation.