题解 | 统计各个部门的工资记录数

select
    C.dept_no,
    C.dept_name,
    count(D.salary) as sum
from
   (
        select
            A.dept_no,
            A.dept_name,
            B.emp_no
        from
            departments A
        left join dept_emp B 
        on A.dept_no = B.dept_no
    ) as C
    left join salaries D 
    on C.emp_no = D.emp_no
group by
    C.dept_no