文章詳細的利用了一個實例來告訴我們如何在mysql刪除數據庫重復記錄的實現方法與原理，有需要的同學可以參考一下下哦。
演示數據
表結構：
代碼如下 復制代碼
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)
數據：
mysql> select * from demo order by id;
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | http://www.bKjia.c0m |
| 2 | |
| 3 | http://www.zhutiai.com |
| 4 | http://www.bKjia.c0m |
| 5 | http://www.zhutiai.com |
+----+------------------------+
5 rows in set (0.00 sec)
當沒有創建表或創建索引權限的時候，可以用下面的方法：
如果你要刪除較舊的重復記錄，可以使用下面的語句：
代碼如下 復制代碼
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.bKjia.c0m |
| 2 | |
| 3 | http://www.zhutiai.com |
+----+------------------------+
3 rows in set (0.00 sec)
你要刪除較新的重復記錄，可以使用下面的語句：
代碼如下 復制代碼
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 | |
| 4 | http://www.bKjia.c0m |
| 5 | http://www.zhutiai.com |
+----+------------------------+
3 rows in set (0.00 sec)
你可以用下面的語句先確認將被刪除的重復記錄：
代碼如下 復制代碼
mysql> SELECT a.*
-> FROM demo a, demo b
-> WHERE a.id > b.id
-> AND (a.site = b.site);
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | http://www.bKjia.c0m |
| 3 | http://www.zhutiai.com |
+----+------------------------+
2 rows in set (0.00 sec)
如果有創建索引的權限，可以用下面的方法：
在表上創建唯一鍵索引：
代碼如下 復制代碼
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.bKjia.c0m |
| 2 | |
| 3 | http://www.zhutiai.com |
+----+------------------------+
3 rows in set (0.00 sec)
重復記錄被刪除後，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
如果有創建表的權限，可以用下面的方法：
創建一個新表，然後將原表中不重復的數據插入新表：
代碼如下 復制代碼
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.bKjia.c0m |
| 2 | |
| 3 | http://www.zhutiai.com |
| 4 | http://www.bKjia.c0m |
| 5 | http://www.zhutiai.com |
+----+------------------------+
5 rows in set (0.00 sec)
mysql> select * from demo_new order by id;
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | http://www.bKjia.c0m |
| 2 | |
| 3 | http://www.zhutiai.com |
+----+------------------------+
3 rows in set (0.00 sec)
然後將原表備份，將新表重命名為當前表：
代碼如下 復制代碼
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.bKjia.c0m |
| 2 | |
| 3 | http://www.zhutiai.com |
+----+------------------------+
3 rows in set (0.00 sec)
注意：使用這種方式創建的表會丟失原表的索引信息！
代碼如下 復制代碼
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; 來查看原表的創建語句，然後使用原表的創建語句創建新表，接著使用 insert … select 語句插入數據，再重命名表即可。
