oracle 取指定数据记录

取前十条记录：
select * from tablename where rownum<10;  //伪列 注意：rownum始终从第一个取值。

取中间记录：如:10到20之间的记录

select * from (select rownum rn,t.* from tablename t) temp where temp.rn between 10 and 20;


查找重复数据：

select * from tablename where id in (select id from tablename group by id having count(id) > 1)