Which statement best describes the main purpose of a primary key?

Which statement best describes the main purpose of a primary key?

• A. Uniquely identifies each row and cannot be null.
• B. Stores auxiliary data about a row.
• C. Can be duplicated.
• D. Is always a foreign key.

Answer: (A)

The main idea is that a primary key uniquely identifies each row and cannot be null. This means every record has a distinct value for the primary key, so you can locate, update, or reference that exact row with confidence. It also serves as the anchor for relationships between tables—other tables can refer to this row by storing its value as a foreign key, linking related data cleanly. If a primary key could be missing or duplicated, you’d lose the ability to distinguish records and maintain consistent relationships across the database.

Storing auxiliary data about a row isn’t the role of a primary key; that kind of information lives in other columns that describe the entity. The idea that it can be duplicated is incorrect because the primary key must be unique for every row. And it isn’t necessarily a foreign key, though other tables may reference it via foreign keys; the primary key itself is not required to be a foreign key.