自己的一点心得，记录一下。

select distinct sal from emp order by sal desc;
select sal,rownum rn from (select distinct sal from emp order by sal desc) where rownum < 4
select sal from (select sal,rownum rn from (select distinct sal from emp order by sal desc) where rownum < 4) where rn > 2
select * from emp where sal = (select sal from (select sal,rownum rn from (select distinct sal from emp order by sal desc) where rownum < 4) where rn > 2)
/*
要求：在员工表中查询出工资第三高（去重）的员工信息：
1. 去重查询工资；
2. 从得到的结果中查询出前三条记录；
3. 从到的结果中查询出第三条记录的工资值；
4. 根据3中得到的值，从员工表中查询出员工信息。
*/