What is a candidate key, and how does it differ from a primary key?

• A. Candidate keys are all minimal attributes that could uniquely identify a row; the primary key is the chosen candidate key used for identification.
• B. Candidate keys are always used as the primary key.
• C. The primary key is never a candidate key.
• D. Candidate keys and primary keys are identical concepts.

Answer: (A)

A candidate key is a minimal set of attributes that can uniquely identify each row in a table. You can have several candidate keys in a table, because more than one combination of attributes might uniquely distinguish records, and each candidate key is minimal (you can’t drop any attribute from it without losing the ability to identify uniquely).

The primary key is one of those candidate keys that you select to be the main identifier for the table. It’s the key you reference in relationships with other tables and is typically defined with strong constraints (often NOT NULL) to ensure reliable identification. Other candidate keys, while still unique, are not the primary key and are considered alternate keys.

So the statement is correct: candidate keys are all minimal attributes that could uniquely identify a row; the primary key is the chosen candidate key used for identification. The other ideas—that candidate keys must always be used as the primary, or that the primary key cannot be a candidate key, or that they are identical—don’t hold, because a table can have multiple candidate keys and only one is designated as the primary key.