Which statement correctly describes primary keys, foreign keys, candidate keys, and superkeys?

• A. A superkey must be minimal.
• B. A primary key uniquely identifies a row; a foreign key references a primary key in another table; candidate keys are the minimal keys; a superkey may include extra attributes.
• C. A candidate key never must be unique.
• D. A foreign key is the same as a primary key.

Answer: (B)

Understanding how different keys relate in a relational database helps explain why this answer is correct. A primary key uniquely identifies a row within its table and cannot be null, serving as the main identifier for that record. A foreign key is a field (or set of fields) that references the primary key in another table, establishing a relationship between the two tables. Candidate keys are the minimal sets of attributes that could serve as a primary key; each candidate key uniquely identifies a row, and none of its proper subsets does. A superkey is any set of attributes that uniquely identifies a row, which may include extra attributes beyond the minimal set.

So the statement fits: the primary key uniquely identifies a row; a foreign key references a primary key in another table; candidate keys are the minimal unique keys; and a superkey may include extra attributes. The other ideas don’t fit because a superkey isn’t required to be minimal, a candidate key must be unique, and a foreign key is not the same as a primary key.