mysql - KEY 关键字是什么意思？

Question

在此 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)
);

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，那么使用 KEY 创建的这种类型的索引有何特别之处？

Answer 1

引用自create-table - indexes andkeys

{INDEX|KEY}

因此KEY通常是INDEX

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.