题解 | 统计各个部门的工资记录数

select a.dept_no,b.dept_name,sum(a.sum) as sum 
from
(select t.*,d.dept_no from 
(select emp_no,count(emp_no) as sum
from salaries
group by  emp_no)t
left join dept_emp d on t.emp_no=d.emp_no)a
left join departments b on a.dept_no=b.dept_no
group by a.dept_no,b.dept_name
order by dept_no