找出所有員工當前(to_date='9999-01-01')具體的薪水salary情況求妹，對于相同的薪水只顯示一次,并按照逆序顯示
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：考察分組操作，這里有兩種做法
使用group by：注意這里的order by 必須放在group by后面汹忠，不然會報錯淋硝。
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;
