方法：3表内连接
select d.dept_no,d.dept_name,count(s.salary) as sum
from departments as d
inner join dept_emp as de
on d.dept_no=de.dept_no
inner join salaries as s
on de.emp_no=s.emp_no
group by d.dept_no;

在进行连接时最好先确定好连接后的字段有哪些