oracle

1：请问oracle 怎么删除重复记录
2：假设员工表中有一字段，薪资，现在要查出这表的所有信息，然后按薪资从高到底排列，前三行显示薪资排名前三的信息，其余的按原来的顺序排列。请举例说明，谢谢。oracle 数据库我不是很熟悉。

select * from employee where rownum <= 3 order by salary desc
union select * from employee t where not exist (select employee_id from employee t2 where t1.employee_id = t2.employee_id and rownum <= 3);