select de.dept_no,d.dept_name,t.title,count(t.emp_no) as count
from departments d 
join dept_emp de on d.dept_no =de.dept_no
join titles t on de.emp_no = t.emp_no
group by de.dept_no,d.dept_name,t.title

order by de.dept_no,t.title

注意group by 里不仅要加de.dept_no,d.dept_name，还有t.title