select 
t1.department department
,round(avg(t2.normal_salary - t2.dock_salary),3) avg_salary
from staff_tb t1
join salary_tb t2
on t1.staff_id = t2.staff_id
where (t2.normal_salary - t2.dock_salary) between 4000 and 30000
group by 1
order by 2 desc