MySql-如何查询删除数据表重复记录

查询 SELECT name, email, COUNT (*)FROM usersGROUP BY name, emailHAVING 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

查询

<code class=" hljs oxygene"><span class="hljs-keyword">SELECT</span>    name, email, COUNT<span class="hljs-comment">(*)FROM    usersGROUP BY    name, emailHAVING     COUNT(*)</span> > <span class="hljs-number">1</span></code>

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：

<code class=" hljs asciidoc"><span class="hljs-header">mysql> desc demo;+-------+------------------+------+-----+---------+----------------+</span><span class="hljs-header">| Field | Type             | Null | Key | Default | Extra          |+-------+------------------+------+-----+---------+----------------+</span>| id    | int(11) unsigned | NO   | PRI | NULL    | auto<span class="hljs-emphasis">_increment || site  | varchar(100)     | NO   | MUL |         |                |+-------+------------------+------+-----+---------+----------------+2 rows in set (0.00 sec)</span></code>

数据：

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

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：

<code class=" hljs haml">mysql> delete from a     -<span class="ruby">> using demo as a, demo as b</span>    -<span class="ruby">> where (a.id > b.id)</span>    -<span class="ruby">> <span class="hljs-keyword">and</span> (a.site = b.site);</span>Query OK, 2 rows affected (0.12 sec)</code>

<code class=" hljs asciidoc"><span class="hljs-header">mysql> select * from demo order by id;+----+------------------------+</span><span class="hljs-header">| id | site                   |+----+------------------------+</span>|  1 | http://www.CodeBit.cn  ||  2 | http://YITU.org        |<span class="hljs-header">|  3 | http://www.ShuoWen.org |+----+------------------------+</span>3 rows in set (0.00 sec)</code>

如果你要删除较新的重复记录，可以使用下面的语句：

<code class=" hljs haml">mysql> delete from a     -<span class="ruby">> using demo as a, demo as b</span>    -<span class="ruby">> where (a.id < b.id)</span>    -<span class="ruby">> <span class="hljs-keyword">and</span> (a.site = b.site);</span>Query OK, 2 rows affected (0.12 sec)</code>

<code class=" hljs asciidoc"><span class="hljs-header">mysql> select * from demo order by id;+----+------------------------+</span><span class="hljs-header">| id | site                   |+----+------------------------+</span>|  2 | http://YITU.org        ||  4 | http://www.CodeBit.cn  |<span class="hljs-header">|  5 | http://www.ShuoWen.org |+----+------------------------+</span>3 rows in set (0.00 sec)</code>

你可以用下面的语句先确认将被删除的重复记录：

<code class=" hljs asciidoc">mysql> SELECT a.* <span class="hljs-code">    -> FROM demo a, demo b</span><span class="hljs-code">    -> WHERE a.id > b.id</span><span class="hljs-header">    -> AND (a.site = b.site);+----+------------------------+</span><span class="hljs-header">| id | site                   |+----+------------------------+</span>|  1 | http://www.CodeBit.cn  |<span class="hljs-header">|  3 | http://www.ShuoWen.org |+----+------------------------+</span>2 rows in set (0.00 sec)</code>

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：

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

<code class=" hljs asciidoc"><span class="hljs-header">mysql> select * from demo order by id;+----+------------------------+</span><span class="hljs-header">| id | site                   |+----+------------------------+</span>|  1 | http://www.CodeBit.cn  ||  2 | http://YITU.org        |<span class="hljs-header">|  3 | http://www.ShuoWen.org |+----+------------------------+</span>3 rows in set (0.00 sec)</code>

重复记录被删除后，如果需要，可以删除索引：

<code class=" hljs avrasm">mysql> alter table demo drop index ukey<span class="hljs-comment">;</span>Query OK, <span class="hljs-number">3</span> rows affected (<span class="hljs-number">0.37</span> <span class="hljs-keyword">sec</span>)<span class="hljs-label">Records:</span> <span class="hljs-number">3</span>  Duplicates: <span class="hljs-number">0</span>  Warnings: <span class="hljs-number">0</span></code>

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：

<code class=" hljs oxygene">mysql> <span class="hljs-keyword">create</span> table demo_new <span class="hljs-keyword">as</span> <span class="hljs-keyword">select</span> * <span class="hljs-keyword">from</span> demo <span class="hljs-keyword">group</span> <span class="hljs-keyword">by</span> site;Query OK, <span class="hljs-number">3</span> rows affected (<span class="hljs-number">0.19</span> sec)Records: <span class="hljs-number">3</span>  Duplicates: <span class="hljs-number">0</span>  Warnings: <span class="hljs-number">0</span></code>

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

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

<code class=" hljs asciidoc"><span class="hljs-header">mysql> select * from demo_new order by id;+----+------------------------+</span><span class="hljs-header">| id | site                   |+----+------------------------+</span>|  1 | http://www.CodeBit.cn  ||  2 | http://YITU.org        |<span class="hljs-header">|  3 | http://www.ShuoWen.org |+----+------------------------+</span>3 rows in set (0.00 sec)</code>

然后将原表备份，将新表重命名为当前表：

<code class=" hljs asciidoc">mysql> rename table demo to demo<span class="hljs-emphasis">_old, demo_</span>new to demo;Query OK, 0 rows affected (0.04 sec)<span class="hljs-header">mysql> show tables;+----------------+</span><span class="hljs-header">| Tables_in_test |+----------------+</span>| demo           |<span class="hljs-header">| demo_old       |+----------------+</span>2 rows in set (0.00 sec)</code>

<code class=" hljs asciidoc"><span class="hljs-header">mysql> select * from demo order by id;+----+------------------------+</span><span class="hljs-header">| id | site                   |+----+------------------------+</span>|  1 | http://www.CodeBit.cn  ||  2 | http://YITU.org        |<span class="hljs-header">|  3 | http://www.ShuoW<a>本文来源gao*daima.com搞@代#码&网6</a>en.org |+----+------------------------+</span>3 rows in set (0.00 sec)</code>

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 asciidoc"><span class="hljs-header">mysql> desc demo;+-------+------------------+------+-----+---------+-------+</span><span class="hljs-header">| Field | Type             | Null | Key | Default | Extra |+-------+------------------+------+-----+---------+-------+</span>| id    | int(11) unsigned | NO   |     | 0       |       |<span class="hljs-header">| site  | varchar(100)     | NO   |     |         |       |+-------+------------------+------+-----+---------+-------+</span>2 rows in set (0.00 sec)</code>

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。

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