SELECT  de.emp_no,
        dm.emp_no AS manager_no,
        s1.salary AS emp_salary,
        s2.salary AS manager_salary
FROM dept_emp de LEFT JOIN dept_manager dm
ON de.dept_no=dm.dept_no
AND de.emp_no<>dm.emp_no
LEFT JOIN salaries s1
ON de.emp_no=s1.emp_no
AND s1.to_date='9999-01-01'
LEFT JOIN salaries s2
ON dm.emp_no=s2.emp_no
AND s2.to_date='9999-01-01'
WHERE s1.salary>s2.salary;

一共3张表，用到3次连接，其中薪资表用到两次
1、dept_emp与dept_manager连接 此处称t1
找出每个emp所在dept的manager_no
2、t1与salaries第一次连接
找出员工emp的薪水
3、t1与salaries第二次连接
找出manager的薪水
4、最后限定条件，emp的薪水比manager高