Oracle实用技巧

4.删除表中重复记录

方法原理:

1、Oracle中，每一条记录都有一个rowid，rowid在整个数据库中是唯一的，　 rowid确定了每条记录是在ORACLE中的哪一个数据文件、块、行上。

2、在重复的记录中，可能所有列的内容都相同，但rowid不会相同，所以只要确定出重复记录中那些具有最大rowid的就可以了，其余全部删除。

实现方法:

SQL> create table a(bm char(4),mc varchar2(20));

Table created

SQL> insert into a values('1111','aaaa');

SQL> insert into a values('1112','aaaa');

SQL> insert into a values('1113','aaaa');

SQL> insert into a values('1114','aaaa');

SQL> insert into a select * from a;

4 rows inserted

SQL> commit;

Commit complete

SQL> select rowid,bm,mc from a;

ROWID BM MC

------------------ ---- --------------------

AAAIRIAAQAAAAJqAAA 1111 aaaa

AAAIRIAAQAAAAJqAAB 1112 aaaa

AAAIRIAAQAAAAJqAAC 1113 aaaa

AAAIRIAAQAAAAJqAAD 1114 aaaa

AAAIRIAAQAAAAJqAAE 1111 aaaa

AAAIRIAAQAAAAJqAAF 1112 aaaa

AAAIRIAAQAAAAJqAAG 1113 aaaa

AAAIRIAAQAAAAJqAAH 1114 aaaa

8 rows selected

查出重复记录

SQL> select rowid,bm,mc from a where a.rowid!=(select max(rowid) from a b where a.bm=b.bm and a.mc=b.mc);

ROWID BM MC

------------------ ---- --------------------

AAAIRIAAQAAAAJqAAA 1111 aaaa

AAAIRIAAQAAAAJqAAB 1112 aaaa

AAAIRIAAQAAAAJqAAC 1113 aaaa

AAAIRIAAQAAAAJqAAD 1114 aaaa

删除重复记录

SQL> delete from a a where a.rowid!=(select max(rowid) from a b where a.bm=b.bm and a.mc=b.mc);

删除4个记录.

SQL> select rowid,bm,mc from a;

ROWID BM MC

------------------ ---- -----

AAAIRIAAQAAAAJqAAE 1111 aaaa

AAAIRIAAQAAAAJqAAF 1112 aaaa

AAAIRIAAQAAAAJqAAG 1113 aaaa

AAAIRIAAQAAAAJqAAH 1114 aaaa