database/_615.sql

SELECT d1.pay_month, d1.department_id,
CASE WHEN d1.department_avg > c1.company_avg THEN 'higher'
     WHEN d1.department_avg < c1.company_avg THEN 'lower'
     ELSE 'same'
END AS 'comparison'
FROM ((SELECT LEFT(s1.pay_date, 7) pay_month, e1.department_id, AVG(s1.amount) department_avg
FROM salary s1
JOIN employee e1 ON s1.employee_id = e1.employee_id
GROUP BY pay_month, e1.department_id) d1
LEFT JOIN (SELECT LEFT(pay_date, 7) pay_month, AVG(amount) company_avg
FROM salary
GROUP BY pay_month) c1 ON d1.pay_month = c1.pay_month)
ORDER BY pay_month DESC, department_id;