What is an example of a one-to-one relationship?

A one-to-one relationship in a database context refers to a situation where a single record in one table is associated with a single record in another table. This means that for every instance of the first entity, there is exactly one corresponding instance of the second entity.

In the case of airline passenger details and seat allocation, this reflects a one-to-one relationship because each passenger typically occupies one specific seat on a flight. Thus, each passenger record corresponds directly to one seat record, establishing a direct pairing without duplication. This exemplifies the nature of one-to-one relationships, where each entity has a unique counterpart.

In contrast, the other scenarios do not fulfill the criteria for a one-to-one relationship. For instance, employees can have multiple phone extension records, and university students can receive multiple grades for different subjects, indicating a potential one-to-many relationship instead. Similarly, many customers can place multiple orders, which illustrates another one-to-many relationship. Understanding these distinctions helps clarify how entities interact within a database structure.