1.分析
建立三个表之间的联系，然后以dept_name分组，以dept_no排序。

2.代码
SELECT d1.dept_no, d1.dept_name, COUNT(dept_name) AS sum
FROM departments AS d1, dept_emp AS d2, salaries AS s
WHERE d1.dept_no = d2.dept_no
and d2.emp_no = s.emp_no
GROUP BY dept_name
ORDER BY dept_no