Grouping with GROUP BY

GROUP BY groups rows together for aggregate calculations.

Count employees per department:

SELECT department_id, COUNT(*)
FROM employees
GROUP BY department_id;

Average salary per department:

SELECT department_id, AVG(salary)
FROM employees
GROUP BY department_id;

Rule: Every column in SELECT must be either:

• In GROUP BY, or
• Inside an aggregate function (COUNT, SUM, etc.)