思路
1、两表连接获得每个员工的薪水
2、按部门分组求最大值(排序求第一也可)
3、找出薪水和部门最大值相等的员工

select
dept_no
,emp_no
,maxSalary
from
(
select
d.emp_no
,salary
,dept_no
,first_value(salary)over(partition by dept_no order by salary desc) maxSalary
from dept_emp d
left join salaries s
on d.emp_no = s.emp_no
) a
where salary = maxSalary