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
users
GROUP BY
name, email
HAVING
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.ShuoWen.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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