查找所有员工自入职以来的薪水涨幅情况

需要分别找到入职时的薪水和当前薪水。入职时的薪水，可以通过employees表中的hire_date和salaries中的from_date来确定，当前薪水通过to_date='9999-01-01'来确定，再连接两个表，相减可得出薪水涨幅，最后使用order by排序

代码块
SELECT e.emp_no,(s2.salary-s1.salary) AS growth
FROM salaries AS s1 
JOIN employees AS e
ON e.emp_no=s1.emp_no
JOIN 
(
    SELECT emp_no,salary
    FROM salaries
    WHERE to_date='9999-01-01'
) AS s2
ON e.emp_no=s2.emp_no
WHERE e.hire_date=s1.from_date
ORDER BY growth

上述代码还可以继续简化：
直接联结两次salaries表，第一次以入职时间作为筛选条件（e.hire_date=s1.from_date），第二次以现在的时间作为筛选条件（s2.to_date='9999-01-01'）

SELECT e.emp_no,(s2.salary-s1.salary) AS growth
FROM employees AS e
LEFT JOIN salaries AS s1
ON e.emp_no=s1.emp_no
LEFT JOIN salaries AS s2
ON e.emp_no=s2.emp_no
WHERE e.hire_date=s1.from_date
AND s2.to_date='9999-01-01'
ORDER BY growth