What are the Differences and Best Uses for MySQL's Primary, Unique, Index, and Fulltext Indexes?

MySQL Table Index Types: Understanding Primary, Unique, Index, and Fulltext

In MySQL, creating tables requires selecting the appropriate type of index to optimize data access. Each index type has distinct characteristics and usage scenarios.

Primary Key

A PRIMARY index is a unique index that ensures the value in the indexed column(s) is unique across all rows in the table. It acts as the primary way to uniquely identify rows and should be defined on the smallest number of columns that can uniquely determine each row. Similar to a UNIQUE index, it prohibits NULL values in the indexed columns.

Unique Index

A UNIQUE index ensures that the value in the indexed column(s) is unique across all rows in the table, except for NULL values. It also serves as a constraint, preventing data with duplicate non-NULL values from being inserted or updated.

Index

A normal index, simply referred to as KEY or INDEX, does not enforce any uniqueness constraints. It allows for duplicate values in the indexed column(s) and is primarily used to improve data retrieval performance without affecting data restraints.

Fulltext Index

A FULLTEXT index is designed for full-text search operations using the MATCH() / AGAINST() syntax. Unlike other indexes, it is implemented differently and is only used in full-text search scenarios.

Comparison

Index Type	Uniqueness	Data Constraints	Usage
PRIMARY	Unique	Yes, except for NULLs	Primary identifier for rows
UNIQUE	Unique	Yes, for non-NULLs	Constraint and fast record retrieval
INDEX	Non-Unique	No	Performance optimization, fast lookups
FULLTEXT	Unique (in combination)	No	Full-text searches, search optimization

Similarities

All index types have:

• The ability to have multiple columns
• Significance of column order in queries (except for FULLTEXT)

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