select d.dept_no,d.dept_name,t.title,count(t.title) count
from departments d
inner join 
dept_emp de
on d.dept_no=de.dept_no
inner join
titles t
on de.emp_no=t.emp_no
where t.to_date='9999-01-01'
and de.to_date='9999-01-01'
group by d.dept_no,t.title

order by d.dept_no,t.title

考察表连接，以及分组依据可以有多个值