The correct answer is All of these because a primary key in a relational database embodies all the characteristics described in the options. A primary key is a crucial database concept that serves multiple vital functions:
Uniquely identifies each record: This is the fundamental purpose of a primary key. It ensures that every single row (record) in a table can be uniquely distinguished from all others, providing a reliable way to reference specific data.
Auto number is an example of a key: An auto-incrementing integer (often called an auto number or identity column) is a very common and practical method for implementing primary keys. It automatically generates unique, sequential numbers for new records, guaranteeing uniqueness without requiring manual input, and simplifies record identification.
Cannot be repeated: This characteristic is a direct consequence of the primary key's need to uniquely identify each record. If a primary key value could be repeated within a table, then multiple records would share the same identifier, violating the principle of uniqueness. Additionally, primary keys generally cannot contain NULL values (this is known as the entity integrity rule), further ensuring their distinctness and reliability.
Because all three individual statements accurately describe aspects, properties, or common implementations of a primary key, 'All of these' is the most comprehensive and correct answer.