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

where a.salary=a.maxSalary

利用窗口函数，得到每个员工本部门的最高工资，然后通过工资和最高工资相等获得部门工资最高的员工id