What is the definition of a primary key and its constraints in a relational model?

• A. A primary key uniquely identifies each row in a table and enforces entity integrity; it cannot be null and must be unique.
• B. A primary key can be null.
• C. A primary key may duplicate values.
• D. A primary key is the same as a foreign key.

Answer: (A)

The main idea here is that a primary key provides a unique identity for every row in a table and enforces entity integrity. This means each row must have a value in the primary key that is both unique across the table and never null, so you can reliably identify and reference that row. This unique, non-null identity is what other tables use when they establish relationships through foreign keys, linking related data together. The other statements stray from how primary keys work: they cannot be null, they cannot have duplicate values, and a primary key is not the same thing as a foreign key (foreign keys point to a primary key in another table to create a relationship).