数据库SQL实战（牛客网）: 汇总各个部门当前员工的title类型的分配数目

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);

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