MySQL KEY关键字是什么意思？
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What does the KEY keyword mean?
提问by MartinodF
In this MySQL table definition:
在这个 MySQL 表定义中：
CREATE TABLE groups (
ug_main_grp_id smallint NOT NULL default '0',
ug_uid smallint default NULL,
ug_grp_id smallint default NULL,
KEY (ug_main_grp_id)
);
What does the KEYkeyword mean? It's not a primary key, it's not a foreign key, so is it just an index? If so, what is so special about this type of index created with KEY?
什么是KEY关键字是什么意思？它不是主键，也不是外键，那么它只是一个索引吗？如果是这样，这种类型的索引有KEY什么特别之处？
回答by MartinodF
Quoting from http://dev.mysql.com/doc/refman/5.1/en/create-table.html
引自http://dev.mysql.com/doc/refman/5.1/en/create-table.html
{INDEX|KEY}
So KEYis an INDEX;)
所以KEY是一个INDEX;）
回答by sergtk
KEYis normally a synonym for INDEX. The key attribute PRIMARY KEYcan also be specified as just KEYwhen given in a column definition. This was implemented for compatibility with other database systems.
KEY通常是 的同义词INDEX。key 属性PRIMARY KEY也可以KEY在列定义中指定时指定。这是为了与其他数据库系统兼容而实施的。
column_definition:
data_type [NOT NULL | NULL] [DEFAULT default_value]
[AUTO_INCREMENT] [UNIQUE [KEY] | [PRIMARY] KEY]
...
Ref: http://dev.mysql.com/doc/refman/5.1/en/create-table.html
参考：http: //dev.mysql.com/doc/refman/5.1/en/create-table.html
