Analysis of how MySQL eliminates duplicate rows

This article mainly introduces MySQL some methods to eliminate duplicate rows. Friends in need can refer to the

sql statement

/*
MySQL 消除重复行的一些方法
---Chu Minfei
---2010-08-12 22:49:44.660
--引用转载请注明出处:http://blog.csdn.NET/feixianxxx
*/
----------------全部字段重复------------------------
 --1使用表替换来删除重复项
 create table test_1(id int,value int);
 insert test_1 select 1,2 union all select 1,2 union all select 2,3;
 --建立一个和源表结构一样的空的临时表
 create table tmp like test_1;
 --向临时表插入不重复的记录
 insert tmp select distinct * from test_1;
 --删除原表
 drop table test_1;
 --更改临时表名为目标表
 rename table tmp to test_1;
 --显示
 mysql> select * from test_1;
+------+-------+
| id  | value |
+------+-------+
|  1 |   2 |
|  2 |   3 |
+------+-------+
 --2.添加auto_increment属性列(这个方法只能用于MyISAM或者BDB引擎的表)
 create table test_1(id int,value int) engine=MyISAM;
 insert test_1 select 1,2 union all select 1,2 union all select 2,3;
 alter table test_1 add id2 int not null auto_increment,
 add primary key(id,value,id2);
 select * from test_1;
+----+-------+-----+
| id | value | id2 |
+----+-------+-----+
| 1 |   2 |  1 |
| 1 |   2 |  2 |
| 2 |   3 |  1 |
+----+-------+-----+
  delete from test_1 where id2<>1;
  alter table test_1 drop id2;
  select * from test_1;
  +----+-------+
| id | value |
+----+-------+
| 1 |   2 |
| 2 |   3 |
+----+-------+
-------------------部分字段重复---------------------
--1.加索引的方式
 create table test_2(id int,value int);
 insert test_2 select 1,2 union all select 1,3 union all select 2,3;
 Alter IGNORE table test_2 add primary key(id);
 select * from test_2;
 +----+-------+
| id | value |
+----+-------+
| 1 |   2 |
| 2 |   3 |
+----+-------+
 我们可以看到 1 3 这条记录消失了 
 我们这里也可以使用Unique约束 因为有可能列中有NULL值，但是这里NULL就可以多个了..
 --2.联合表删除
 create table test_2(id int,value int);
 insert test_2 select 1,2 union all select 1,3 union all select 2,3;
 delete A from test_2 a join (select MAX(value) as v ,ID from test_2 group by id) b
 on a.id=b.id and a.value<>b.v;
 select * from test_2;
 +------+-------+
| id  | value |
+------+-------+
|  1 |   3 |
|  2 |   3 |
+------+-------+
--3.使用Increment_auto也可以就是上面全部字段去重的第二个方法
--4.容易错误的方法
--有些朋友可能会想到子查询的方法，我们来试验一下
 create table test_2(id int,value int);
 insert test_2 select 1,2 union all select 1,3 union all select 2,3;
 delete a from test_2 a where exists(select * from test_2 where a.id=id and a.value<value);
 /*ERROR 1093 (HY000): You can&#39;t specify target table &#39;a&#39; for update in FROM clause*/
 
 目前，您不能从一个表中删除，同时又在子查询中从同一个表中选择。
 
 
 ------------------删除特定重复行--------------
 --主要通过order by +limit 或者直接limit 
 create table test_3(id int,value int);
 insert test_3 select 1,2 union all select 1,3 union all select 1,4 union all select 2,3;
 --这是要保留ID=1 value最小的那个记录，删除其他id为的记录
 delete from test_3 where id=1 order by value desc limit 2;
 select * from test_3;
+------+-------+
| id  | value |
+------+-------+
|  1 |   2 |
|  2 |   3 |
+------+-------+
 如果你只想删除任意的记录 保留一条 就可以去掉order by

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