题解 | #统计各个部门的工资记录数#

1、返回dept_no，dept_name，sum：
（1）departments表中的dept_no，dept_name
（2）salaries表中求出sum：COUNT(salary) AS sum
2、通过dept_emp连接departments表和salaries表：
SELECT demp.dept_no, COUNT(salary) AS sum
FROM dept_emp AS demp INNER JOIN salaries
ON demp.emp_no = salaries.emp_no
GROUP BY dept_no
3、连接departments表和dept_emp表和salaries表：
FROM departments AS d LEFT JOIN
(SELECT demp.dept_no, COUNT(salary) AS sum
FROM dept_emp AS demp INNER JOIN salaries
ON demp.emp_no = salaries.emp_no
GROUP BY dept_no) AS j
WHERE d.dept_no = j.dept_no
4、按照dept_no排序：ORDER BY d.dept_no
5、串联：
SELECT d.dept_no, d.dept_name, j.sum
FROM departments AS d LEFT JOIN
(SELECT demp.dept_no, COUNT(salary) AS sum
FROM dept_emp AS demp INNER JOIN salaries
ON demp.emp_no = salaries.emp_no
GROUP BY dept_no) AS j
WHERE d.dept_no = j.dept_no
ORDER BY d.dept_no