MySql-如何查询删除数据表重复记录

查询

SELECT
    name, email, COUNT(*)
FROM
 users
GROUP BY
 name, email
HAVING 
 COUNT(*) > 1

重点来了，查询容易，那应该如何删除重复记录呢？

演示数据
表结构：

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)

数据：

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)

当没有创建表或创建索引权限的时候，可以用下面的方法：

如果你要删除较旧的重复记录，可以使用下面的语句：

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)

如果你要删除较新的重复记录，可以使用下面的语句：

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)

你可以用下面的语句先确认将被删除的重复记录：

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)

如果有创建索引的权限，可以用下面的方法：

在表上创建唯一键索引：

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)

重复记录被删除后，如果需要，可以删除索引：

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

如果有创建表的权限，可以用下面的方法：

创建一个新表，然后将原表中不重复的数据插入新表：

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)

然后将原表备份，将新表重命名为当前表：

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：使用这种方式创建的表会丢失原表的索引信息！

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; 来查看原表的创建语句，然后使用原表的创建语句创建新表，接着使用 insert … select 语句插入数据，再重命名表即可。