What does the KEY keyword mean?

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)
);

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?

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Answers:

Method 1

Method 2

Answers:

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Method 1

Quoting from create-table – indexes andkeys

{INDEX|KEY}

So KEY is usually an 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.

Method 2

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.

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

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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