8.找出所有员工当前(to_date='9999-01-01')

找出所有员工当前(to_date='9999-01-01')具体的薪水salary情况，对于相同的薪水只显示一次,并按照逆序显示

CREATE TABLE `salaries` (
`emp_no` int(11) NOT NULL,
`salary` int(11) NOT NULL,
`from_date` date NOT NULL,
`to_date` date NOT NULL,
PRIMARY KEY (`emp_no`,`from_date`));

解析：考察分组操作，这里有两种做法
使用group by：注意这里的order by 必须放在group by后面，不然会报错。

select salary
from salaries
where to_date = '9999-01-01'
group by salary
order by salary desc;

使用distinct:

select distinct salary
from salaries
where to_date='9999-01-01'
order by salary desc;