In a relational table, which constraint ensures that no two rows have the same value for a key attribute?

• A. Primary Key
• B. Foreign Key
• C. Unique Constraint
• D. Index

Answer: (A)

Key values in a table row must uniquely identify that row. A primary key provides that guarantee by enforcing that every row has a distinct value for the key and, by design, disallows nulls. This combination ensures no two rows share the same key value and every row can be uniquely identified.

A unique constraint also enforces uniqueness, but in many databases it may allow nulls, so it doesn't always guarantee a non-null unique value for every row. A foreign key, on the other hand, enforces a relationship to another table, not uniqueness within the same table. An index speeds up searches but does not by itself enforce uniqueness unless it’s defined as unique.