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Snailclimb merged 1 commit into from
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Update mysql-index.md #1568

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2 changes: 1 addition & 1 deletion docs/database/mysql/mysql-index.md
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Expand Up @@ -94,7 +94,7 @@ InnoDB 引擎中，其数据文件本身就是索引文件。相比 MyISAM，索

一张数据表有只能有一个主键，并且主键不能为 null，不能重复。

在 MySQL 的 InnoDB 的表中，当没有显示的指定表的主键时，InnoDB 会自动先检查表中是否有唯一索引的字段，如果有，则选择该字段为默认的主键，否则 InnoDB 将会自动创建一个 6Byte 的自增主键。
在 MySQL 的 InnoDB 的表中，当没有显示的指定表的主键时，InnoDB 会自动先检查表中是否有唯一索引且不允许存在null值的字段，如果有，则选择该字段为默认的主键，否则 InnoDB 将会自动创建一个 6Byte 的自增主键。

### 二级索引(辅助索引)

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