Which rule defines Boyce-Codd Normal Form (BCNF) and how is it stronger than 3NF?

• A. BCNF allows non-key dependencies as long as they do not violate dependencies.
• B. BCNF is the same as 3NF in terms of rules.
• C. BCNF requires every determinant to be a candidate key; it is stricter than 3NF.
• D. BCNF only applies to single-valued attributes.

Answer: (C)

BCNF hinges on a clear rule about the determinants of dependencies: for every non-trivial functional dependency X -> Y that holds in the relation, X must be a candidate key. A candidate key is a minimal set of attributes that uniquely identifies a row, so when the determinant is a candidate key, the dependency is tied to a key rather than to a non-key attribute. This eliminates dependencies that rely on non-key attributes determining other attributes, which helps prevent update anomalies.

This rule makes BCNF stricter than 3NF. In 3NF, you can have a dependency where the left side isn’t a key as long as the right side is a prime attribute (part of some candidate key) or the left side is a superkey. That allowance means 3NF can still permit certain non-key–driven dependencies, whereas BCNF would require decomposing the relation to remove them. In practice, you’d split the relation so that all remaining dependencies have determinants that are keys, resulting in a design with fewer anomalies.