What Does the `KEY` Keyword Really Do in MySQL Table Definitions?

KEY Keyword in MySQL Table Definition

The MySQL table definition snippet provided introduces a KEY keyword. This syntax often raises questions about its significance.

Meaning of the KEY Keyword

Contrary to popular belief, a KEY keyword alone does not explicitly define a primary key or foreign key. Instead, it serves the purpose of creating an index.

According to the MySQL documentation, "KEY is normally a synonym for INDEX." This indicates that KEY is typically used to create a standard index.

Types of Indexes Created with KEY

The use of KEY primarily creates a STANDARD B-Tree index, which is the default type of index in MySQL. These indexes optimize query performance by sorting data in a balanced, tree-like structure.

Difference from Other Indexes

While KEY is frequently used to create standard indexes, it should be noted that there are other types of indexes that can be created with different keywords, such as:

• PRIMARY KEY: Creates a unique, not null primary key constraint.
• FOREIGN KEY: Creates a foreign key constraint, referencing another table.
• UNIQUE INDEX or UNIQUE KEY: Creates a unique index, allowing duplicate values within the column but not within the index.
• FULLTEXT or FULLTEXT INDEX: Creates a full-text index optimized for fast text searches.

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