比较容易理解的思路

每个部门中员工最大，就是该员工薪水必须大于等于他所在部门员工的最高工资。

select a.dept_no,a.emp_no,b.salary maxSalary
from dept_emp a
left join salaries b on a.emp_no = b.emp_no
where b.salary >=
(select max(bt.salary) from dept_emp at
left join salaries bt
on at.emp_no = bt.emp_no
where at.dept_no = a.dept_no)
order by a.dept_no