select
  d.dept_no,
  d.dept_name,
  count(salary) sum
from
  departments d
  join dept_emp de on d.dept_no = de.dept_no
  join salaries s on s.emp_no = de.emp_no
group by
  dept_no
order by
  dept_no

还是考察的表连接

选择连接方式的时候还是有点不确定