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<ol class="chapter"><li class="chapter-item expanded affix "><a href="index.html">README</a></li><li class="chapter-item expanded "><a href="1-SQL基础知识.html">SQL基础知识</a></li><li class="chapter-item expanded "><a href="2-MySQL安装.html">MySQL安装</a></li><li class="chapter-item expanded "><a href="3-MySQL管理.html">MySQL管理</a></li><li class="chapter-item expanded "><a href="4-数据类型.html">数据类型</a></li><li class="chapter-item expanded "><a href="5-创建数据表.html">创建数据表</a></li><li class="chapter-item expanded "><a href="6-删除数据表.html">删除数据表</a></li><li class="chapter-item expanded "><a href="7-修改数据.html">修改数据</a></li><li class="chapter-item expanded "><a href="8-查询数据表.html">查询数据表</a></li><li class="chapter-item expanded "><a href="9-MySQL数据处理.html">MySQL数据处理</a></li><li class="chapter-item expanded "><a href="10-MySQL事务.html">MySQL事务</a></li><li class="chapter-item expanded "><a href="11-建立索引.html" class="active">建立索引</a></li><li class="chapter-item expanded "><a href="12-临时表.html">临时表</a></li><li class="chapter-item expanded "><a href="13-自增序列.html">自增序列</a></li><li class="chapter-item expanded "><a href="14-重复数据.html">重复数据</a></li><li class="chapter-item expanded "><a href="15-MySQL常用函数.html">MySQL常用函数</a></li></ol>
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<h2 id="建立索引"><a class="header" href="#建立索引">建立索引</a></h2>
<p>索引时，你需要确保该索引是应用在 SQL 查询语句的条件(一般作为 WHERE 子句的条件)。</p>
<h3 id="普通索引"><a class="header" href="#普通索引">普通索引</a></h3>
<h4 id="创建索引"><a class="header" href="#创建索引">创建索引</a></h4>
<p>这是最基本的索引，它没有任何限制。它有以下几种创建方式：</p>
<pre><code class="language-sql">CREATE INDEX indexName ON mytable(username(length));
</code></pre>
<p>如果是CHAR，VARCHAR类型，length可以小于字段实际长度；如果是BLOB和TEXT类型，必须指定 length。</p>
<h4 id="修改表结构添加索引"><a class="header" href="#修改表结构添加索引">修改表结构(添加索引)</a></h4>
<pre><code class="language-sql">ALTER table tableName ADD INDEX indexName(columnName)
</code></pre>
<h4 id="删除索引的语法"><a class="header" href="#删除索引的语法">删除索引的语法</a></h4>
<pre><code class="language-sql">DROP INDEX [indexName] ON mytable;
</code></pre>
<h3 id="唯一索引"><a class="header" href="#唯一索引">唯一索引</a></h3>
<p>它与前面的普通索引类似，不同的就是：索引列的值必须唯一，但允许有空值。如果是组合索引，则列值的组合必须唯一。它有以下几种创建方式：</p>
<h4 id="创建索引-1"><a class="header" href="#创建索引-1">创建索引</a></h4>
<pre><code class="language-sql">CREATE UNIQUE INDEX indexName ON mytable(username(length))
</code></pre>
<h4 id="修改表结构"><a class="header" href="#修改表结构">修改表结构</a></h4>
<pre><code class="language-sql">ALTER table mytable ADD UNIQUE [indexName] (username(length))
</code></pre>
<h3 id="使用alter-命令添加和删除索引"><a class="header" href="#使用alter-命令添加和删除索引">使用ALTER 命令添加和删除索引</a></h3>
<pre><code class="language-sql">mysql> ALTER TABLE testalter_tbl ADD INDEX (c);
</code></pre>
<p>你还可以在 ALTER 命令中使用 DROP 子句来删除索引。尝试以下实例删除索引:</p>
<pre><code class="language-sql">mysql> ALTER TABLE testalter_tbl DROP INDEX c;
</code></pre>
<h3 id="使用-alter-命令添加和删除主键"><a class="header" href="#使用-alter-命令添加和删除主键">使用 ALTER 命令添加和删除主键</a></h3>
<p>主键只能作用于一个列上，添加主键索引时，你需要确保该主键默认不为空（NOT NULL）。实例如下：</p>
<pre><code class="language-sql">mysql> ALTER TABLE testalter_tbl MODIFY i INT NOT NULL;
mysql> ALTER TABLE testalter_tbl ADD PRIMARY KEY (i);
</code></pre>
<p>你也可以使用 ALTER 命令删除主键：</p>
<pre><code class="language-sql">mysql> ALTER TABLE testalter_tbl DROP PRIMARY KEY;
</code></pre>
<p>删除主键时只需指定PRIMARY KEY，但在删除索引时，你必须知道索引名。</p>
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