A Level Computer Science OCR Practice Exam

In an ERM one-to-one relationship, how does one entity relate to another?

1. One entity can correspond to several entities

2. One entity can correspond to one and only one other entity

3. One entity can correspond to one or more entities

4. It can relate to none of the entities

The correct answer is: One entity can correspond to one and only one other entity

In an Entity-Relationship Model (ERM), a one-to-one relationship means that one entity is associated with exactly one instance of another entity. This definition is crucial in database design because it helps maintain clear relationships that enforce data integrity. When one entity corresponds to one and only one other entity, it ensures that for each unique instance of the first entity, there is a distinct and singular pairing with the second entity. This kind of relationship might be observed in scenarios such as a person having exactly one passport, and each passport being assigned to exactly one person. Thus, if you think about it practically, the association indicates exclusivity which is what defines the one-to-one relationship. Other options don't accurately capture the essence of a one-to-one relationship. For instance, suggesting that one entity can correspond to several entities implies a one-to-many relationship, which is fundamentally different. Similarly, stating that one entity can correspond to one or more entities introduces ambiguity that contradicts the exclusive nature characteristic to one-to-one relationships. Lastly, implying that one entity can relate to none of the entities overlooks the premise that there exists a relationship being defined, even if it’s one-to-one. Therefore, relating exactly one entity to another is the hallmark of a one-to-one relationship.