ORACLE开发：Oracle去除重复数据
查询某些字段相同的记录
如：查询col1与col2值相同的记录：
select a.* from table1 a, table1 b where a.id <> b.id and a.col1 = b.col1 and a.col2 = b.col2;
一、用rowid方法：
根据oracle自带的rowid属性进行判断是否存在重复记录。
rowid伪列用于唯一标识物理位置的表行，当用insert插入数据时，会自动生成rowid,与数据一起存放，形如：AAAL=XAAAEAAAAA。
1、查数据：
select * from table1 a where rowid!=
(select max(rowid) from table1 b where a.col1 = b.col1 and a.col2 = b.col2;
(select max(rowid) from table1 b where a.col1 = b.col1 and a.col2 = b.col2;
2、删数据：
保留rowid最大的记录：
delete from table1 a where rowid!=
(select max(rowid) from table1 b where a.col1 = b.col1 and a.col2 = b.col2;
(select max(rowid) from table1 b where a.col1 = b.col1 and a.col2 = b.col2;
二、group by 方法：
1、查数据：
select * from table1 a where (a.col1,a.col2) in
(select col1,col2 from table1 group by col1,col2 having count(*)>1)
(select col1,col2 from table1 group by col1,col2 having count(*)>1)
2、删数据：
删除表中多余的重复记录（多个字段），只保留rowid最小的记录。
delete from table1 a where (a.col1,a.col2) in
(select col1,col2 from table1 group by col1,col2 having count(*)>1)
and rowid not in
(select min(rowid) from table1 group by col1,col2 having count(*)>1)
(select col1,col2 from table1 group by col1,col2 having count(*)>1)
and rowid not in
(select min(rowid) from table1 group by col1,col2 having count(*)>1)
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