GROUP BY & HAVING

GROUP BY splits rows into groups by a column and applies the aggregate to each group. “How many employees per department?” is exactly a GROUP BY.

SELECT dept_id, COUNT(*) AS staff, AVG(salary) AS avg_salary
FROM employees
GROUP BY dept_id;

WHERE filters rows before grouping; HAVING filters groups after grouping:

SELECT status, COUNT(*) AS orders
FROM orders
GROUP BY status
HAVING COUNT(*) >= 3;