select  dept_no,emp_no,salary
from(
    select dept_no,emp_no,salary,max(salary)over(partition by dept_no) maxSalary
    from dept_emp left join salaries using(emp_no)
    where dept_emp.to_date="9999-01-01"
)t1
where salary = maxSalary

窗口查出部门最大，外层再筛选