Answer
GROUP BY combines rows that have equal grouping values into groups. • Aggregate functions can calculate one result per group. • Selected non-aggregate expressions generally must be grouping expressions. • Grouping happens after WHERE filtering.
💡 SQL Example
SELECT department_id, COUNT(*) FROM employees GROUP BY department_id;
Result
10 | 4
20 | 3
⚡ Quick Revision
GROUP BY creates groups for aggregate calculations.