题解 | 统计各个部门平均薪资

select department,round(avg(normal_salary-dock_salary),3) avg_salary
from staff_tb i
left join salary_tb s on i.staff_id=s.staff_id
where s.normal_salary-s.dock_salary >=4000 and s.normal_salary-s.dock_salary<=30000
group by department
order by avg_salary desc

！：where和having的区别