What are the Key Differences Between PRIMARY, UNIQUE, INDEX, and FULLTEXT Indexes in MySQL?

Understanding MySQL Index Types: PRIMARY, UNIQUE, INDEX, and FULLTEXT

MySQL provides several index types that enhance database performance. This article explains the key differences between the four commonly used indexes: PRIMARY, UNIQUE, INDEX, and FULLTEXT.

PRIMARY Index

Similar to a UNIQUE index, a PRIMARY index ensures the uniqueness of all rows in the index. However, it has the following unique characteristics:

• There can only be one PRIMARY index per table.
• It is typically named 'PRIMARY' and provides a primary means of identifying rows.
• It should be on the smallest number of columns that uniquely define a row.
• Some database systems store table data within the PRIMARY index's B-tree structure.

UNIQUE Index

A UNIQUE index enforces the uniqueness of rows within the specified columns. It:

• Prevents identical non-NULL values in multiple rows.
• Allows for NULL values, potentially allowing multiple rows with NULLs to be identical.
• Can be used for data restraints by preventing duplicate records.

INDEX (Non-Unique)

A basic INDEX or KEY type refers to a non-unique index. It:

• Allows non-distinct values in the index.
• Does not enforce any data constraints.
• Is used solely for faster data access and retrieval.

FULLTEXT Index

Unlike the previous types, a FULLTEXT index is specialized for full-text search functionality:

• Indexes the full text content of columns, such as strings.
• Facilitates efficient searching using the MATCH() / AGAINST() clause.
• Does not follow the same column ordering rules as other index types.

Similarities

Despite their differences, all of these index types share the following characteristics:

• Can contain multiple columns.
• Require column order to match query criteria, starting from the leftmost column.

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