主要考察了 多表链接 先 表1左链接表2形成新表4 ，然后表3左链接表4 再进行分组统计即可

select
b.dept_no,
b.dept_name,
count(b.salary) as sum
from
(select
s.emp_no,
s.salary,
a.dept_no,
a.dept_name
from salaries s left join 
(select a.dept_no,a.dept_name,b.emp_no,b.from_date,b.to_date from departments a left join dept_emp b on a.dept_no=b.dept_no)a on s.emp_no=a.emp_no)b
group by dept_no 
order by dept_no