數據庫SQL實戰（牛客網）: 彙總各個部門當前員工的title類型的分配數目

彙總各個部門當前員工的title類型的分配數目，結果給出部門編號dept_no、dept_name、其當前員工所有的title以及該類型title對應的數目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;