In the Acme Auto example, why does the Maintenance entity contain the vehicle ID as a field?

The Maintenance entity contains the vehicle ID as a field primarily to join the Vehicle and Maintenance data types. This relationship allows for clear connections between maintenance records and the specific vehicles they pertain to. By including the vehicle ID in the Maintenance entity, it establishes a way to reference the Vehicle entity directly, facilitating the retrieval of all maintenance activities associated with a particular vehicle. This joining helps maintain data integrity and ensures that queries related to vehicle maintenance can accurately reflect which maintenance records correspond to which vehicles.

In data modeling, this kind of relationship is essential for maintaining structures that represent real-world scenarios, such as tracking maintenance history, scheduling future service, or analyzing vehicle performance over time, all of which rely on this connectedness through a shared identifier.