Subqueries with IN

Subqueries work great with IN for matching multiple values.

Find employees in high-paying departments:

SELECT * FROM employees
WHERE department_id IN (
    SELECT department_id FROM employees
    GROUP BY department_id
    HAVING AVG(salary) > 80000
);

The subquery returns a list of department IDs, then IN checks each employee.

NOT IN finds employees NOT in those departments.