What's the Difference Between Primary Keys and Unique Keys in MySQL?

Detailed explanation of the difference between MySQL primary key and unique key

In a MySQL database, understanding the difference between primary keys and unique keys is critical to optimizing data management. This article will delve into the differences between the two and their application scenarios.

Primary Key

The primary key assigns a unique identifier to each row in the table to ensure its uniqueness. Key features of primary keys include:

• Uniqueness: Each table has only one primary key.
• Non-null values: In MySQL, primary keys usually cannot contain null values, and NOT NULL constraints are enforced.
• Unique Identification: It is the only means of uniquely identifying records in a table.

Unique key

Unlike primary keys, a table can have multiple unique keys. These keys provide additional mechanisms to ensure data integrity:

• Multiple occurrences: Multiple unique keys can exist in the same table at the same time.
• Allow NULL values: Unique keys allow NULL values, allowing recorded key columns to have no values.
• Candidate Key: A unique key can also be a candidate key and may become an alternative primary key.
• Potential non-uniqueness: Unique keys can contain null values, which allows multiple rows to contain the same null value, resulting in non-uniqueness.

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