两张表解决

select uni.dept_no, uni.emp_no, max_salary.salary
from
    (select d.dept_no, s.emp_no, s.salary
     from dept_emp d join salaries s 
     on d.emp_no = s.emp_no 
     and d.to_date = '9999-01-01'
     and s.to_date = '9999-01-01'   
    ) as uni, /* 部门编号,员工编号,当前薪水 */
    (select d.dept_no, max(s.salary) as salary
     from dept_emp d join salaries s 
     on d.emp_no = s.emp_no
     and d.to_date = '9999-01-01'
     and s.to_date = '9999-01-01'   
     group by d.dept_no
    ) as max_salary /* 部门编号,当前最高薪水 */
where uni.salary = max_salary.salary
and uni.dept_no = max_salary.dept_no
order by uni.dept_no;

一张表记录最高薪水（部门编号,当前最高薪水），一张表记录所有员工的部门及薪水信息（部门编号,员工编号,当前薪水），用部门编号和薪水相等取到最高薪水的员工ID。