MySql-如何查询删除数据表重复记录_MySQL

查询

<code class="hljs oxygene">SELECT
    name, email, COUNT(*)
FROM
    users
GROUP BY
    name, email
HAVING 
    COUNT(*) > 1</code>

<strong>重点来了，查询容易，那应该如何删除重复记录呢？</strong>

演示数据<br> 表结构：

<code class="hljs oxygene"><code class="hljs asciidoc">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)</code></code>

<code class="hljs asciidoc">数据：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc">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)</code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc">当<strong>没有创建表或创建索引权限</strong>的时候，可以用下面的方法：

<code class="hljs asciidoc"><code class="hljs asciidoc">如果你要<strong>删除较旧的重复记录</strong>，可以使用下面的语句：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml">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)</code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">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)</code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">如果你要<strong>删除较新的重复记录</strong>，可以使用下面的语句：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml">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)</code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">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)</code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc">你可以用下面的语句<strong>先确认将被删除的重复记录</strong>：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc">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)</code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc">如果<strong>有创建索引的权限</strong>，可以用下面的方法：

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc">在表上创建唯一键索引：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm">mysql> alter ignore table demo add unique index ukey (site);
Query OK, 5 rows affected (0.46 sec)
Records: 5  Duplicates: 2  Warnings: 0</code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc">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)</code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc">重复记录被删除后，如果需要，可以删除索引：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm">mysql> alter table demo drop index ukey;
Query OK, 3 rows affected (0.37 sec)
Records: 3  Duplicates: 0  Warnings: 0</code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm">如果<strong>有创建表的权限</strong>，可以用下面的方法：

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm">创建一个新表，然后将原表中不重复的数据插入新表：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene">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</code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc">mysql> show tables;
+----------------+
| Tables_in_test |
+----------------+
| demo           |
| demo_new       |
+----------------+
2 rows in set (0.00 sec)</code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc">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)</code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">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)</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">然后将原表备份，将新表重命名为当前表：

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">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)</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">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)</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">注意：使用这种方式创建的表会丢失原表的索引信息！

<code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">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)</code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code></code>

<code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs haml"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs asciidoc"><code class="hljs avrasm"><code class="hljs oxygene"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc"><code class="hljs asciidoc">如果要保持和原表信息一致，你可以使用 <strong>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</strong>，接着使用 insert … select 语句插入数据，再重命名表即可。