SELECT his.emp_no,today.salary-his.salary
FROM
-- 求出所有员工最早入职工资
(SELECT emp_no,MIN(from_date),salary FROM salaries s GROUP by emp_no) his,
-- 求出在职的员工当前工资
(SELECT emp_no,salary,s.to_date FROM salaries s WHERE  s.to_date = '9999-01-01') today
WHERE his.emp_no = today.emp_no
ORDER BY today.salary-his.salary asc