思路：用部门员工关系表为主表连接另外两表，然后按部门，title分组计数
select
d1.dept_no
,dept_name
,title
,count(*) count
from dept_emp d1
left join departments d2 on d1.dept_no = d2.dept_no
left join titles t on d1.emp_no = t.emp_no
group by 1,2,3
order by dept_no