牛客网数据库SQL实战26—— 汇总各个部门当前员工的title类型的分配数目

牛客网数据库SQL实战26—— 汇总各个部门当前员工的title类型的分配数目

题目描述

汇总各个部门当前员工的title类型的分配数目，结果给出部门编号dept_no、dept_name、其当前员工所有的title以及该类型title对应的数目count

CREATE TABLE `departments` (
`dept_no` char(4) NOT NULL,
`dept_name` varchar(40) NOT NULL,
PRIMARY KEY (`dept_no`));

CREATE TABLE `dept_emp` (
`emp_no` int(11) NOT NULL,
`dept_no` char(4) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`dept_no`));

CREATE TABLE IF NOT EXISTS `titles` (
`emp_no` int(11) NOT NULL,
`title` varchar(50) NOT NULL,
`from_date` date NOT NULL,
`to_date` date DEFAULT NULL);

输入描述:

无

输出描述:

dept_no	dept_name	title	count
d001	Marketing	Senior Engineer	1
d001	Marketing	Staff	1
d002	Finance	Senior Engineer	1
d003	Human Resources	Senior Staff	1
d004	Production	Senior Engineer	2
d005	Development	Senior Staff	1
d006	Quality Management	Engineer	2
d006	Quality Management	Senior Engineer	1

我的解答

select d.dept_no,d.dept_name,t.title,count(*) as count
from departments d
left join dept_emp de
on d.dept_no = de.dept_no
left join titles t
on de.emp_no = t.emp_no
where de.to_date = '9999-01-01'
and t.to_date = '9999-01-01'
group by d.dept_no,d.dept_name,t.title

三表连接，按照dept_no和title分组计数

我觉得最好的答案

本题的关键在于用 GROUP BY 同时对 de.dept_no 和 t.title 进行分组，具体思路如下：

1. 先用 INNER JOIN 连接 dept_emp 与 salaries，根据测试数据添加限定条件 de.to_date = ‘9999-01-01’ AND t.to_date = ‘9999-01-01’，即当前员工的当前头衔
2. 再用 INNER JOIN 连接departments，限定条件为 de.dept_no = dp.dept_no，即部门编号相同
3. 最后用 GROUP BY 同时对 de.dept_no 和 t.title 进行分组，用 COUNT(t.title) 统计相同部门下相同头衔的员工个数

SELECT de.dept_no, dp.dept_name, t.title, COUNT(t.title) AS count
FROM titles AS t INNER JOIN dept_emp AS de 
ON t.emp_no = de.emp_no AND de.to_date = '9999-01-01' AND t.to_date = '9999-01-01'
INNER JOIN departments AS dp 
ON de.dept_no = dp.dept_no
GROUP BY de.dept_no, t.title