select dept_no, count(salary) as sum
from salaries left outer join dept_emp
on salaries.emp_no = dept_emp.emp_no
group by dept_no;

select ds.dept_no, dept_name, sum
from departments,
(select dept_no, count(salary) as sum
from salaries left outer join dept_emp
on salaries.emp_no = dept_emp.emp_no
group by dept_no) as ds
where departments.dept_no = ds.dept_no
order by ds.dept_no;

只要是逻辑思考完毕，这些题真的就是费时间，但不困难了。

但是时间，或者说效率，是我亟需提高的。