What are the common types of cardinality in relationships?

• A. One-to-one, one-to-many, and many-to-many.
• B. One-to-one only.
• C. Many-to-one only.
• D. Cardinality has no impact on database design.

Answer: (C)

Cardinality in relationships describes how many records in one table relate to records in another. The common patterns are one-to-one, one-to-many, and many-to-many. One-to-one means each row in the first table relates to exactly one row in the second, and vice versa. One-to-many means a single row in the first table can relate to many rows in the second, while each of those second-row records links back to only that one first-row record. Many-to-many means rows in both tables can relate to multiple rows in the other, which usually requires an intermediate junction table to implement.

This concept matters for database design because the pattern determines how you structure keys and foreign keys. For example, a one-to-many relationship places a foreign key on the “many” side; a many-to-many relationship needs an associative table to break the many-to-many link into two one-to-many relationships. The idea that there’s only one type, or that cardinality doesn’t affect design, doesn’t reflect how relationships are modeled in practice.