What Does the KEY Keyword Mean in MySQL Indexing?

Understanding the Meaning of KEY Keyword in MySQL

In MySQL, the KEY keyword is often encountered in table definitions. However, its purpose and distinction from other index types can be confusing.

Definition of KEY

As mentioned in the quoted documentation from "create-table - indexes and keys," KEY generally serves as a synonym for INDEX. It defines an index on a specific column or columns without assigning a specific role, such as a primary or foreign key.

KEY as an Index

Therefore, KEY essentially creates an index on the designated column(s). An index optimizes data retrieval by allowing the database to quickly locate rows based on the indexed values. This speeds up queries that search or filter using those values.

Special Aspects of KEY-Created Indexes

While KEY creates a standard index, it does not confer any additional properties or features beyond a basic index. It does not enforce uniqueness or reference integrity like primary or foreign keys.

Example

In the example provided:

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)
);

The KEY on ug_main_grp_id creates a standard index, enabling efficient retrieval of groups based on their main group ID.

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