# 按要求进行group by和count
SELECT t.dept_no,t.dept_name,count(1) as sum 
FROM
(
# 先进行计算所得到的连表信息获取所有符合的薪资信息
SELECT
d.dept_name,
de.dept_no,
s.emp_no,
s.salary
FROM
departments d,
dept_emp de,
salaries s
WHERE
d.dept_no = de.dept_no
AND
de.emp_no = s.emp_no
)t
GROUP BY dept_no,dept_name
ORDER BY dept_no