select c.dept_no,c.dept_name,count(d.salary) from 
(
    select a.dept_no,a.dept_name,b.emp_no 
    from departments a 
    left join dept_emp b 
    on a.dept_no=b.dept_no
) c left join salaries d
on c.emp_no=d.emp_no
group by c.dept_no
order by c.dept_no
;