MySQL資料庫中刪除重複記錄的方法總結[推薦]

           表結構：
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)

資料：
mysql> select * from demo order by id;
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | |
| 2 | |
| 3 | |
| 4 | |
| 5 | |
+----+------------------------+
5 rows in set (0.00 sec)

當沒有建立表或建立索引許可權的時候，可以用下面的方法：

如果你要刪除較舊的重複記錄，可以使用下面的語句：
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 | |
| 2 | |
| 3 | |
+----+------------------------+
3 rows in set (0.00 sec)

如果你要刪除較新的重複記錄，可以使用下面的語句：
mysql> delete from a
-> using demo as a, demo as b
-> where (a.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 | |
| 5 | |
+----+------------------------+
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複記錄：
mysql> SELECT a.*
-> FROM demo a, demo b
-> WHERE a.id > b.id
-> AND (a.site = b.site);
+----+------------------------+
| id | site |
+----+------------------------+
| 1 | |
| 3 | |
+----+------------------------+
2 rows in set (0.00 sec)

如果有建立索引的許可權，可以用下面的方法：

在表上建立唯一鍵索引：

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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 | | | 2 | | | 3 | | +----+------------------------+ 3 rows in set (0.00 sec)

重複記錄被刪除後，如果需要，可以刪除索引：

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

如果有建立表的許可權，可以用下面的方法：

建立一個新表，然後將原表中不重複的資料插入新表：

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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 | | | 2 | | | 3 | | | 4 | | | 5 | | +----+------------------------+ 5 rows in set (0.00 sec)  mysql> select * from demo_new order by id; +----+------------------------+ | id | site | +----+------------------------+ | 1 | | | 2 | | | 3 | | +----+------------------------+ 3 rows in set (0.00 sec)

然後將原表備份，將新表重新命名為當前表：

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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 | | | 2 | | | 3 | | +----+------------------------+ 3 rows in set (0.00 sec)

注意：使用這種方式建立的表會丟失原表的索引資訊！

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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)

如果要保持和原表資訊一致，你可以使用 show create table demo; 來檢視原表的建立語句，然後使用原表的建立語句建立新表，接著使用 insert … select 語句插入資料，再重新命名錶即可。

當然，如果要避免重複記錄，最好的辦法還是不要插入重複資料，可以參考本站另外一篇文章：MySQL 當記錄不存在時插入