分页：
SELECT * FROM (
SELECT A.*, ROWNUM RN
FROM
(SELECT * FROM
TABLE_NAME) A
WHERE ROWNUM <= 40)
WHERE RN >= 21
去重：
delete from 表名 t1
where rowid<(select max(rowid)
from
表名 t2
where t1.user_name=t2.user_name)
Oracle清空回收站命令：
purge recyclebin