题解 | #统计各个部门的工资记录数#

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