方法一：

思路：对工资去重，降序

代码：

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

方法二：

思路：用 group By 去重对工资去重，再降序。 group by 去重的性能高于 distinct

代码：

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