*将两张小表连接为新表 t，salaries表和新表 t 连接
select t.dept_no,t.dept_name,count(t.dept_name) as sum
from salaries s,(select de.emp_no,de.dept_no,d.dept_name
                 from dept_emp de,departments d
                 where de.dept_no = d.dept_no) as t
where s.emp_no = t.emp_no
group by t.dept_name
order by t.dept_no