Which statement best describes candidate keys?

• A. Candidate keys are the only keys that can uniquely identify a row.
• B. Candidate keys are always the primary key.
• C. Candidate keys cannot be used for identification.
• D. Candidate keys include every attribute in the table.

Answer: (D)

Candidate keys are minimal sets of attributes that can uniquely identify each row in a table. There can be more than one candidate key, and one of them is typically chosen as the primary key. A candidate key is a special kind of superkey, but not every superkey is a candidate key because it must be minimal—no attribute can be removed without losing the unique identification property.

The statement that candidate keys include every attribute in the table isn’t generally true. A candidate key is usually a subset of attributes; it only contains all attributes when that full set is the smallest combination that still uniquely identifies a row, which is not the usual case. It’s also incorrect to say candidate keys are the only keys that can uniquely identify a row, since larger superkeys with extra attributes can also do the job. And candidate keys can indeed be used for identification, so that claim isn’t correct either.