大佬们，可以帮我看看为什么这个思路为什么输出后的结果是两行呢？不进行联结表他不是很需要性能支撑吗？如果数据较大是不是楼主这思路会导致消耗很大的性能呢？联结表会优于楼主这个思路吗？请麻烦大佬们指点 select a.emp_no ,b.manager_no ,a.emp_salary ,b.manager_salary from ( select s.emp_no, dm.dept_no,s.salary as emp_salary from salaries as s left join dept_manager as dm on dm.emp_no=s.emp_no left join dept_emp as de on dm.dept_no=de.dept_no where dm.dept_no is null ) as a , ( select dm.emp_no as manager_no,dm.dept_no, s.salary as manager_salary from salaries as s left join dept_manager as dm on dm.emp_no=s.emp_no left join dept_emp as de on dm.dept_no=de.dept_no ) as b where a.emp_salary>b.manager_salary;