What is a derived table?

SELECT dept_id, avg_salary FROM (SELECT dept_id, AVG(salary) AS avg_salary FROM employees GROUP BY dept_id) AS dept_avg WHERE avg_salary > 50000;

What is 'dept_avg'?

SELECT d.dept_name, stats.emp_count FROM departments d JOIN (SELECT dept_id, COUNT(*) AS emp_count FROM employees GROUP BY dept_id) AS stats ON d.id = stats.dept_id;

What is being joined?

SELECT * FROM (SELECT *, ROW_NUMBER() OVER (PARTITION BY dept_id ORDER BY salary DESC) AS rn FROM employees) AS ranked WHERE rn <= 3;

What does this find?

What is a limitation of derived tables?