What is the KEY Keyword in MySQL and How Does it Differ from Primary and Foreign Keys?

Understanding the KEY Keyword in MySQL

In the context of SQL databases, the KEY keyword is used to create an index on a specific column or set of columns. While indices are commonly associated with primary and foreign keys, the KEY keyword can introduce a unique type of index.

Defining an Index with KEY

An index is a data structure that enhances the performance of data retrieval by allowing the database to quickly locate rows matching a given search criterion. The KEY keyword creates an index without specifying any additional properties, such as whether it should be unique or clustered.

Benefits of a KEY Index

A KEY index offers several benefits, including:

• Improved Search Speed: The index allows the database to skip the full table scan and access the data more efficiently, resulting in faster search results.
• Data Integrity: Indices help ensure the uniqueness of data in a column, preventing duplicate entries.
• Query Optimization: The database optimizer can utilize the KEY index to determine the most efficient execution plan for queries involving the indexed column.

Difference from Primary and Foreign Keys

A KEY index differs from a primary key, which uniquely identifies each row in a table, and a foreign key, which establishes a relationship between two tables. The KEY index is simply an index that aids in the efficient retrieval of data.

Syntax

The syntax for creating a KEY index in MySQL is:

KEY (column_list)

where column_list is a comma-separated list of columns to be indexed.

Example

Consider the following 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)
);

In this example, the KEY index is created on the ug_main_grp_id column, improving the performance of queries that search or filter based on this column.

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