SQL8

 题目描述:
查找所有员工自入职以来的薪水涨幅情况(to_date='9999-01-01')
给出员工编号emp_no以及其对应的薪水涨幅growth，并按照growth进行升序

    CREATE TABLE `employees` (
    `emp_no` int(11) NOT NULL,
    `birth_date` date NOT NULL,
    `first_name` varchar(14) NOT NULL,
    `last_name` varchar(16) NOT NULL,
    `gender` char(1) NOT NULL,
    `hire_date` date NOT NULL COMMENT '入职时间',
    PRIMARY KEY (`emp_no`));

    CREATE TABLE `salaries` (
    `emp_no` int(11) NOT NULL COMMENT '员工编号',
    `salary` int(11) NOT NULL COMMENT '薪水',
    `from_date` date NOT NULL COMMENT '入职时间',
    `to_date` date NOT NULL,
    PRIMARY KEY (`emp_no`,`from_date`));


本题思路是先分别用两次LEFT JOIN左连接employees与salaries，建立两张表，分别存放员工当前工资（CurTable）
与员工入职时的工资（StaTable），再用INNER JOIN连接上述两表，然后限定在同一员工下用当前工资减去入职工资，
得到的是对应的员工的薪水涨幅记录，最后再对

关键点：内层用LEFT JOIN，外层用INNER JOIN 

    select CurTable.emp_no,(CurTable.salary-StaTable.salary) as growth
    from
    (select  e1.emp_no, s1.salary from employees e1 left join salaries s1 on
     e1.emp_no=s1.emp_no and s1.to_date='9999-01-01') as CurTable
    inner join 
    (select  e2.emp_no, s2.salary from employees e2 left join salaries s2 on
     e2.emp_no=s2.emp_no and e2.hire_date=s2.from_date ) as StaTable
    on CurTable.emp_no=StaTable.emp_no order by growth