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关于MySQL中主键 pri mul

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Jun 07, 2016 pm 05:21 PM

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因为虽然索引的多列组合是唯一的，比如ID+NAME是唯一的，但是没一个单独的列依然可以有重复的值，只要ID+NAME是唯一的即可

1. 如果Key是空的, 那么该列值的可以重复, 表示该列没有索引, 或者是一个非唯一的复合索引的非前导列

2. 如果Key是PRI, 那么该列是主键的组成部分

3. 如果Key是UNI, 那么该列是一个唯一值索引的第一列(前导列),并别不能含有空值(NULL)

4. 如果Key是MUL, 那么该列的值可以重复, 该列是一个非唯一索引的前导列(第一列)或者是一个唯一性索引的组成部分但是可以含有空值NULL

如果对于一个列的定义，同时满足上述4种情况的多种，，比如一个列既是PRI,又是UNI

那么"desc 表名"的时候，显示的Key值按照优先级来显示 PRI->UNI->MUL

那么此时，显示PRI

一个唯一性索引列可以显示为PRI,并且该列不能含有空值，同时该表没有主键

一个唯一性索引列可以显示为MUL, 如果多列构成了一个唯一性复合索引

因为虽然索引的多列组合是唯一的，比如ID+NAME是唯一的，但是没一个单独的列依然可以有重复的值

只要ID+NAME是唯一的即可

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