MySQL查看、创建和删除索引的方法

清疚 2022-06-14 02:16 308阅读 0赞

MySQL查看、创建和删除索引的方法。分享给大家供大家参考。具体如下：

**1．索引作用**

在索引列上，除了上面提到的有序查找之外，数据库利用各种各样的快速定位技术，能够大大提高查询效率。特别是当数据量非常大，查询涉及多个表时，使用索引往往能使查询速度加快成千上万倍。

例如，有3个未索引的表t1、t2、t3，分别只包含列c1、c2、c3，每个表分别含有1000行数据组成，指为1～1000的数值，查找对应值相等行的查询如下所示。

SELECT c1,c2,c3 FROM t1,t2,t3 WHERE c1=c2 AND c1=c3  
此查询结果应该为1000行，每行包含3个相等的值。在无索引的情况下处理此查询，必须寻找3个表所有的组合，以便得出与WHERE子句相配的那些行。而可能的组合数目为1000×1000×1000（十亿），显然查询将会非常慢。

如果对每个表进行索引，就能极大地加速查询进程。利用索引的查询处理如下。

（1）从表t1中选择第一行，查看此行所包含的数据。  
（2）使用表t2上的索引，直接定位t2中与t1的值匹配的行。类似，利用表t3上的索引，直接定位t3中与来自t1的值匹配的行。  
（3）扫描表t1的下一行并重复前面的过程，直到遍历t1中所有的行。

在此情形下，仍然对表t1执行了一个完全扫描，但能够在表t2和t3上进行索引查找直接取出这些表中的行，比未用索引时要快一百万倍。  
利用索引，MySQL加速了WHERE子句满足条件行的搜索，而在多表连接查询时，在执行连接时加快了与其他表中的行匹配的速度。

**2.  创建索引**

在执行CREATE TABLE语句时可以创建索引，也可以单独用CREATE INDEX或ALTER TABLE来为表增加索引。

1．ALTER TABLE

ALTER TABLE用来创建普通索引、UNIQUE索引或PRIMARY KEY索引。

[?][Link 1]

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      2 
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      3 
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    <div style="line-height:15.4px; margin:0px!important; padding:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:relative!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ALTER</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">TABLE</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> table_name </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ADD</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">INDEX</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> index_name (column_list)</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ALTER</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">TABLE</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> table_name </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ADD</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">UNIQUE</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> (column_list)</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ALTER</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">TABLE</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> table_name </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ADD</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">PRIMARY</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important"> KEY</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> (column_list)</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

其中table\_name是要增加索引的表名，column\_list指出对哪些列进行索引，多列时各列之间用逗号分隔。索引名index\_name可选，缺省时，MySQL将根据第一个索引列赋一个名称。另外，ALTER TABLE允许在单个语句中更改多个表，因此可以在同时创建多个索引。

2．CREATE INDEX

CREATE INDEX可对表增加普通索引或UNIQUE索引。

[?][Link 1]

table\_name、index\_name和column\_list具有与ALTER TABLE语句中相同的含义，索引名不可选。另外，不能用CREATE INDEX语句创建PRIMARY KEY索引。

**3．索引类型**

在创建索引时，可以规定索引能否包含重复值。如果不包含，则索引应该创建为PRIMARY KEY或UNIQUE索引。对于单列惟一性索引，这保证单列不包含重复的值。对于多列惟一性索引，保证多个值的组合不重复。

PRIMARY KEY索引和UNIQUE索引非常类似。事实上，PRIMARY KEY索引仅是一个具有名称PRIMARY的UNIQUE索引。这表示一个表只能包含一个PRIMARY KEY，因为一个表中不可能具有两个同名的索引。

下面的SQL语句对students表在sid上添加PRIMARY KEY索引。

复制代码代码如下:

ALTER TABLE students ADD PRIMARY KEY (sid)

**4.  删除索引**

可利用ALTER TABLE或DROP INDEX语句来删除索引。类似于CREATE INDEX语句，DROP INDEX可以在ALTER TABLE内部作为一条语句处理，语法如下。

[?][Link 1]

<table style="border-collapse:collapse; border-spacing:0px; border:1px solid rgb(204,204,204); margin-top:0.54em; text-align:center; width:643px; clear:both; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin-right:0px!important; margin-bottom:0px!important; margin-left:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important"> 
 <tbody style="background:none!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; text-align:left!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-size:1em!important; min-height:auto!important"> 
  <tr style="background:none!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; margin:0px!important; outline:0px!important; overflow:visible!important; padding:0px!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-size:1em!important; min-height:auto!important"> 
   <td style="padding:0.2em 0.46em; border:1px solid rgb(204,204,204); margin:0px!important; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important; color:rgb(175,175,175)!important"> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      1 
    </div> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      2 
    </div> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px 0.5em 0px 1em!important; border-width:0px 3px 0px 0px!important; border-top-style:initial!important; border-right-style:solid!important; border-bottom-style:initial!important; border-left-style:initial!important; border-top-color:initial!important; border-right-color:rgb(108,226,108)!important; border-bottom-color:initial!important; border-left-color:initial!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; text-align:right!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important">
      3 
    </div> </td> 
   <td style="padding:0.2em 0.46em; border:1px solid rgb(204,204,204); width:611px; margin:0px!important; background:none!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-size:1em!important; min-height:auto!important"> 
    <div style="line-height:15.4px; margin:0px!important; padding:0px!important; background:none!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:relative!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important"> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">DROP</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">INDEX</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> index_name </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ON</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important">talbe_name</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ALTER</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">TABLE</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> table_name </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">DROP</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">INDEX</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> index_name</code> 
     </div> 
     <div style="line-height:15.4px; margin:0px!important; padding:0px 1em!important; border:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; width:auto!important; min-height:auto!important; white-space:pre!important"> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">ALTER</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">TABLE</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; min-height:auto!important"> table_name </code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">DROP</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important">PRIMARY</code> 
      <code style="margin:3px auto 0px; padding:0px 0px 0px 5px; border-left:3px solid rgb(108,226,108); line-height:20px; width:640px; clear:both; border-top:0px!important; border-right:0px!important; border-bottom:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,&quot;Bitstream Vera Sans Mono&quot;,&quot;Courier New&quot;,Courier,monospace!important; font-weight:bold!important; min-height:auto!important; color:rgb(0,102,153)!important"> KEY</code> 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

其中，前两条语句是等价的，删除掉table\_name中的索引index\_name。  
第3条语句只在删除PRIMARY KEY索引时使用，因为一个表只可能有一个PRIMARY KEY索引，因此不需要指定索引名。如果没有创建PRIMARY KEY索引，但表具有一个或多个UNIQUE索引，则MySQL将删除第一个UNIQUE索引。  
如果从表中删除了某列，则索引会受到影响。对于多列组合的索引，如果删除其中的某列，则该列也会从索引中删除。如果删除组成索引的所有列，则整个索引将被删除。

**5．查看索引**

[?][Link 1]

· Table  
表的名称。  
· Non\_unique  
如果索引不能包括重复词，则为0。如果可以，则为1。  
· Key\_name  
索引的名称。  
· Seq\_in\_index  
索引中的列序列号，从1开始。  
· Column\_name  
列名称。  
· Collation  
列以什么方式存储在索引中。在MySQL中，有值‘A'（升序）或NULL（无分类）。  
· Cardinality  
索引中唯一值的数目的估计值。通过运行ANALYZE TABLE或myisamchk -a可以更新。基数根据被存储为整数的统计数据来计数，所以即使对于小型表，该值也没有必要是精确的。基数越大，当进行联合时，MySQL使用该索引的机会就越大。  
· Sub\_part  
如果列只是被部分地编入索引，则为被编入索引的字符的数目。如果整列被编入索引，则为NULL。  
· Packed  
指示关键字如何被压缩。如果没有被压缩，则为NULL。  
· Null  
如果列含有NULL，则含有YES。如果没有，则该列含有NO。  
· Index\_type  
用过的索引方法（BTREE, FULLTEXT, HASH, RTREE）。  
· Comment

[Link 1]: http://www.jb51.net/article/73372.htm#