What is essential for the integrity of a Primary Key (PK)?

A Primary Key (PK) is an essential component of a relational database that uniquely identifies each record in a table. The primary requirement for a Primary Key is that it cannot include duplicate values. This characteristic is crucial because the primary function of a PK is to ensure that every entry in the table is unique, allowing for accurate data retrieval and maintaining data integrity.

When a PK allows duplicate values, it undermines its purpose and can lead to confusion when querying the database, as there would be no definitive way to identify a specific row. Hence, uniqueness is the foundational attribute that establishes the integrity of the Primary Key.

Regarding the other considerations, while a PK must not allow null values, it can indeed consist of more than one column (which creates a composite key), and its primary purpose is not primarily for performance optimization, but rather for ensuring the integrity and uniqueness of the data.