Which of the following best describes entity integrity?

Entity integrity is a fundamental principle in database design that ensures each entity, or record, in a table is unique and identifiable. This is primarily enforced through the use of primary keys, which serve as unique identifiers for records. When entity integrity is maintained, no two rows in a table can have the same values for the primary key, ensuring that each row can be uniquely distinguished from others.

This principle is crucial for maintaining the reliability and integrity of the database because it prevents duplication and ambiguity in how records are referenced and managed. For instance, if two records could have the same primary key, it would become unclear which record is being referred to in queries, which could lead to inconsistent data retrieval and management difficulties.

In contrast, allowing null values in a primary key would violate the concept of entity integrity, as a null value means that there is no unique identifier available for a record. Similarly, while foreign key constraints ensure that relationships are valid by enforcing that foreign keys refer to existing primary keys, they do not define entity integrity directly. Lastly, allowing multiple instances of the same attribute would compromise the uniqueness required for entity integrity, as it would mean that the distinguishing feature that the primary key provides is lost.