KEY关键字是什么意思?

2021-11-20 00:00:00 indexing mysql

在这个 MySQL 表定义中:

In this MySQL table definition:

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)
);

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，使用 KEY 创建的这种类型的索引有什么特别之处?

What does the KEY keyword 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?

推荐答案

引用自 create-table - 索引和键

{INDEX|KEY}

所以 KEY 通常是一个 INDEX

KEY 通常是 INDEX 的同义词.键属性 PRIMARY KEY 可以在列定义中给出时，也可以仅指定为 KEY.这个实施是为了与其他数据库系统兼容.

KEY is normally a synonym for INDEX. The key attribute PRIMARY KEY can also be specified as just KEY when given in a column definition. This was implemented for compatibility with other database systems.

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