题解 | 查找在职员工自入职以来的薪水涨幅情况

先创建两张表：
	1.初始工资表：用hire_date = from_date作为表连接条件来筛选，工资合同开始日期为该员工进公司的第一天，即该员工在公司最初的工资
	2.当前工资表：用WHERE to_date = '9999-01-01'作为条件，筛选出在职员工的最新合同
接着连接两张表，用当前工资-初始工资，即可得出在职员工的薪资涨幅总和

WITH start as (
    SELECT 
        e.emp_no,
        salary
    FROM
        employees e 
        JOIN salaries s ON e.emp_no = s.emp_no AND e.hire_date = s.from_date
),
current as (
    SELECT
        emp_no,
        salary
    FROM 
        salaries
    WHERE 
        to_date = '9999-01-01'
) 
SELECT 
    s.emp_no,
    c.salary - s.salary as growth
FROM 
    start s JOIN current c USING(emp_no)
ORDER BY 
    growth