表結(jié)構(gòu)：
mysql> desc demo;
+-------+------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+------------------+------+-----+---------+----------------+
| id | int(11) unsigned | NO | PRI | NULL | auto_increment |
| site | varchar(100) | NO | MUL | | |
+-------+------------------+------+-----+---------+----------------+
2 rows in set (0.00 sec)

數(shù)據(jù)：
mysql> select * from demo order by id;
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | http://www.CodeBit.cn |
| 2 | http://YITU.org |
| 3 | http://www.ShuoWen.org |
| 4 | http://www.CodeBit.cn |
| 5 | http://www.ShuoWen.org |
+----+------------------------+
5 rows in set (0.00 sec)

當(dāng)沒(méi)有創(chuàng)建表或創(chuàng)建索引權(quán)限的時(shí)候，可以用下面的方法：

如果你要?jiǎng)h除較舊的重復(fù)記錄，可以使用下面的語(yǔ)句：
mysql> delete from a
-> using demo as a, demo as b
-> where (a.id > b.id)
-> and (a.site = b.site);
Query OK, 2 rows affected (0.12 sec)

mysql> select * from demo order by id;
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | http://www.CodeBit.cn |
| 2 | http://YITU.org |
| 3 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)

如果你要?jiǎng)h除較新的重復(fù)記錄，可以使用下面的語(yǔ)句：
mysql> delete from a
-> using demo as a, demo as b
-> where (a.id < b.id)
-> and (a.site = b.site);
Query OK, 2 rows affected (0.12 sec)

mysql> select * from demo order by id;
+----+------------------------+
| id | site |
+----+------------------------+
| 2 | http://YITU.org |
| 4 | http://www.CodeBit.cn |
| 5 | http://www.ShuoWen.org |
+----+------------------------+
3 rows in set (0.00 sec)

你可以用下面的語(yǔ)句先確認(rèn)將被刪除的重復(fù)記錄：
mysql> SELECT a.*
-> FROM demo a, demo b
-> WHERE a.id > b.id
-> AND (a.site = b.site);
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | http://www.CodeBit.cn |
| 3 | http://www.ShuoWen.org |
+----+------------------------+
2 rows in set (0.00 sec)

如果有創(chuàng)建索引的權(quán)限，可以用下面的方法：

在表上創(chuàng)建唯一鍵索引：

 
mysql> alter ignore table demo add unique index ukey (site); 
Query OK, 5 rows affected (0.46 sec) 
Records: 5 Duplicates: 2 Warnings: 0 

mysql> select * from demo order by id; 
+----+------------------------+ 
| id | site | 
+----+------------------------+ 
| 1 | http://www.CodeBit.cn | 
| 2 | http://YITU.org | 
| 3 | http://www.ShuoWen.org | 
+----+------------------------+ 
3 rows in set (0.00 sec) 

重復(fù)記錄被刪除后，如果需要，可以刪除索引：

 
mysql> alter table demo drop index ukey; 
Query OK, 3 rows affected (0.37 sec) 
Records: 3 Duplicates: 0 Warnings: 0 

如果有創(chuàng)建表的權(quán)限，可以用下面的方法：

創(chuàng)建一個(gè)新表，然后將原表中不重復(fù)的數(shù)據(jù)插入新表：

 
mysql> create table demo_new as select * from demo group by site; 
Query OK, 3 rows affected (0.19 sec) 
Records: 3 Duplicates: 0 Warnings: 0 

mysql> show tables; 
+----------------+ 
| Tables_in_test | 
+----------------+ 
| demo | 
| demo_new | 
+----------------+ 
2 rows in set (0.00 sec) 

mysql> select * from demo order by id; 
+----+------------------------+ 
| id | site | 
+----+------------------------+ 
| 1 | http://www.CodeBit.cn | 
| 2 | http://YITU.org | 
| 3 | http://www.ShuoWen.org | 
| 4 | http://www.CodeBit.cn | 
| 5 | http://www.ShuoWen.org | 
+----+------------------------+ 
5 rows in set (0.00 sec) 

mysql> select * from demo_new order by id; 
+----+------------------------+ 
| id | site | 
+----+------------------------+ 
| 1 | http://www.CodeBit.cn | 
| 2 | http://YITU.org | 
| 3 | http://www.ShuoWen.org | 
+----+------------------------+ 
3 rows in set (0.00 sec) 

然后將原表備份，將新表重命名為當(dāng)前表：

 
mysql> rename table demo to demo_old, demo_new to demo; 
Query OK, 0 rows affected (0.04 sec) 

mysql> show tables; 
+----------------+ 
| Tables_in_test | 
+----------------+ 
| demo | 
| demo_old | 
+----------------+ 
2 rows in set (0.00 sec) 

mysql> select * from demo order by id; 
+----+------------------------+ 
| id | site | 
+----+------------------------+ 
| 1 | http://www.CodeBit.cn | 
| 2 | http://YITU.org | 
| 3 | http://www.ShuoWen.org | 
+----+------------------------+ 
3 rows in set (0.00 sec) 

注意：使用這種方式創(chuàng)建的表會(huì)丟失原表的索引信息！

 
mysql> desc demo; 
+-------+------------------+------+-----+---------+-------+ 
| Field | Type | Null | Key | Default | Extra | 
+-------+------------------+------+-----+---------+-------+ 
| id | int(11) unsigned | NO | | 0 | | 
| site | varchar(100) | NO | | | | 
+-------+------------------+------+-----+---------+-------+ 
2 rows in set (0.00 sec) 

如果要保持和原表信息一致，你可以使用 show create table demo; 來(lái)查看原表的創(chuàng)建語(yǔ)句，然后使用原表的創(chuàng)建語(yǔ)句創(chuàng)建新表，接著使用 insert … select 語(yǔ)句插入數(shù)據(jù)，再重命名表即可。

當(dāng)然，如果要避免重復(fù)記錄，最好的辦法還是不要插入重復(fù)數(shù)據(jù)，可以參考本站另外一篇文章：MySQL 當(dāng)記錄不存在時(shí)插入

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