SELECT st.department	,ROUND(AVG(sa.normal_salary-sa.dock_salary),3)  AS avg_salary
FROM  staff_tb AS st JOIN salary_tb AS sa ON st.staff_id=sa.staff_id
WHERE sa.normal_salary-sa.dock_salary >=4000 AND  sa.normal_salary-sa.dock_salary <= 30000
GROUP BY st.department
ORDER BY  avg_salary DESC