select st.department as department,
    round(avg(sa.normal_salary - sa.dock_salary), 3) as avg_salary

from staff_tb as st 
    left join salary_tb as sa on st.staff_id = sa.staff_id
where (sa.normal_salary - sa.dock_salary) between 4000 and 30000
group by st.department
order by avg_salary desc
#好坑，题目没有讲剔除的薪资是实际薪资还是基本工资，很坑