What restriction does third normal form (3NF) impose on dependencies?

• A. 3NF allows transitive dependencies.
• B. 3NF requires no functional dependencies.
• C. 3NF requires all attributes to depend on the primary key.
• D. 3NF requires no transitive dependencies.

Answer: (D)

Third normal form eliminates transitive dependencies. That means non-prime attributes should not depend on other non-prime attributes via a key. If a key determines A and A then determines B, B ends up depending on the key indirectly, which 3NF prohibits.

For example, a table with StudentID as the key and attributes StudentName, DeptID, DeptName has a transitive dependency if DeptName depends on DeptID, which depends on the key. To satisfy 3NF, you split into two relations: one with StudentID, StudentName, DeptID and another with DeptID, DeptName. Now each non-prime attribute depends directly on a key in its relation.

That’s why the correct restriction is no transitive dependencies.