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

Dive into the difference between primary keys and unique keys

In relational database management systems, the concepts of primary keys and unique keys often appear. These constraints serve specific purposes and exhibit different characteristics, and understanding these characteristics is critical to efficient database design.

Primary Key

At the heart of every table is the primary key, which is a unique identifier that clearly distinguishes each row from every other row. Therefore:

• A table can only have one primary key constraint.
• In some database management systems (DBMS), such as MySQL, the primary key mandates that the value is NOT NULL, ensuring that each row has a non-null value in the specified column.
• The primary key is the cornerstone of data integrity, uniquely identifying each record in the table.

Unique key

Unlike primary keys, unique keys provide an additional level of uniqueness in a table:

• They allow multiple unique key constraints to exist in the same table, providing greater flexibility in data organization.
• Unlike primary keys, unique keys can accommodate null values.
• They can serve as candidate keys, providing a unique identifier if the primary key is not available.
• Null values ​​in a unique key weaken its "uniqueness" since multiple rows may share the same null value.

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