SELECT department_salary.pay_month, department_id,

CASE

WHEN department_avg < company_avg THEN 'lower'

WHEN department_avg > company_avg THEN 'higher'

ELSE 'same'

END AS comparison

(

SELECT department_id, AVG(amount) AS department_avg, date_format(pay_date, '%Y-%m') AS pay_month

FROM salary JOIN employee ON salary.employee_id = employee.employee_id

GROUP BY department_id, pay_month

) AS department_salary

(

SELECT AVG(amount) AS company_avg, date_format(pay_date, '%Y-%m') AS pay_month

FROM salary

GROUP BY date_format(pay_date, '%Y-%m')

) AS company_salary

ON department_salary.pay_month = company_salary.pay_month

;