select a.dept_no, a.emp_no,a.salary from
(select d.dept_no, d.emp_no,s.salary,
row_number () over(partition by d.dept_no order by s.salary desc) as ranking

from dept_emp d 
join salaries s on d.emp_no = s.emp_no)a
where a.ranking =1

order by a.dept_no

不难，但要注意细节