What is the Purpose of the KEY Keyword in MySQL Table Definitions?

Understanding the KEY Keyword in MySQL Tables

In the realm of MySQL database creation, the KEY keyword has sparked considerable curiosity. This article will shed light on the meaning of KEY within MySQL table definitions, addressing the recurring question of its purpose.

What is the KEY Keyword?

The KEY keyword in MySQL serves as a means to define an index on a table column. An index, in database terminology, acts as a guide to quickly locate rows with specific values, significantly improving query performance.

KEY vs. PRIMARY KEY vs. INDEX

It's essential to clarify that KEY is not synonymous with PRIMARY KEY or INDEX. A PRIMARY KEY represents a unique identifier for each row in a table, ensuring data integrity. An INDEX is a more general term referring to any index created on a table column.

Advantages of KEY

The specific type of index created with KEY is known as a "normal" index. While normal indices provide performance benefits similar to other index types, they lack certain limitations.

Normal indices can be built on non-unique columns, allowing multiple rows to possess the same indexed value. This allows for faster lookup based on multiple search criteria. Furthermore, normal indices do not require explicit definition of the index type (e.g., B-tree) and are created with MySQL's default index type, which is typically B-tree.

Usage of KEY

The KEY keyword is commonly employed in conjunction with a column name to specify which column the index should be applied to. For instance:

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, an index is created on the ug_main_grp_id column. Note that KEY is used synonymously with INDEX in this context.

In Conclusion

KEY in MySQL table definitions denotes an index on a specific column. Normal indices created with KEY offer performance benefits while maintaining flexibility in index type and allowing for non-unique values. Understanding the nuances of KEY is crucial for optimizing MySQL table performance and ensuring efficient data retrieval.

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