What are aggregate functions in SQL?

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Aggregate functions compute a single value from a set of rows. Often used with GROUP BY to summarize per-group; without grouping, they collapse the entire result set to one row. COUNT — number of rows; COUNT(*) counts all, COUNT(col) ignores NULLs. SUM — total of numeric values; ignores NULLs. AVG — arithmetic mean. MIN / MAX — smallest / largest value. GROUP_CONCAT — concatenates values into a delimited string (MySQL-specific). Ignore NULLs — all aggregates skip NULLs except COUNT(*) . Know all 6: COUNT, SUM, AVG, MIN, MAX, GROUP_CONCAT. COUNT(*) vs COUNT(column): * includes NULLs, column excludes NULLs. GROUP_CONCAT is MySQL-specific (often asked). All aggregates ignore NULLs (except COUNT(*)).