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

思路：看创建的表以及结果：tiltle是惟一的，需要的是 departments表和title表里面的dept_no，dept_name和title以及title的重复计数，但是departments和title表没有直接关系，
1.连接dept_emp和titles表，判断to_data=‘9999-01-01’ ，d.emp_no=t.emp_no
2.连接department和1操作之后的表，条件dept_no==
3.group by dept_no,title

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

select dp.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 t.to_date = '9999-01-01'and de.to_date = '9999-01-01'
inner join departments as dp
on de.dept_no = dp.dept_no
group by t.title ，de.dept_no   --group by 后面的顺序修改，输出顺序不同，不是输出描述中所需要的表，导致出错不通过