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:
- 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
- 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
- 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