Which statement correctly distinguishes candidate keys from superkeys?

• A. Candidate keys are always larger than superkeys.
• B. Every superkey is a candidate key.
• C. Candidate keys are minimal sets of attributes that can uniquely identify a row; a superkey is any set of attributes that uniquely identifies a row.
• D. A primary key cannot be a candidate key.

Answer: (C)

The important distinction is minimality versus just uniqueness. A superkey is any combination of attributes that uniquely identifies a row. A candidate key is a superkey, but a minimal one: you can’t remove any attribute from a candidate key without losing the ability to uniquely identify the row.

So, every candidate key is a superkey, but not every superkey is a candidate key. A relation can have several candidate keys, and one of them (or more) can be chosen as the primary key. For example, if both StudentID and Email uniquely identify a row, {StudentID} and {Email} are candidate keys, while {StudentID, Email} is a superkey that isn’t minimal and thus isn’t a candidate key.