Primary Key vs. Unique Key in MySQL: What's the Difference?

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

In database table design, the choice of key constraints is crucial to data integrity and performance. In MySQL, there are two main types of key constraints: primary key and unique key. It's crucial to understand the difference between them.

Primary Key

Primary key constraints serve as unique identifiers for each row in the table. Its main features include:

• Single instance: Only one primary key can exist in each table.
• Non-null value: MySQL automatically adds the NOT NULL attribute to the primary key and does not allow null values.
• Unique identifier: The primary key establishes a unique identity for each row to ensure data integrity.

Unique key

Unique key constraints restrict duplicate values ​​in a specific column or set of columns. It has similarities and differences with primary keys:

• Multiple instances: Unlike primary keys, multiple unique keys can be created for a single table.
• Nulls: Unique keys allow nulls, meaning multiple rows can share the same null value and not be considered unique.
• Candidate key: Although it is not a primary key, if the unique key meets the uniqueness and non-null value conditions, it can become a primary key candidate.

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