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The figure shows three object tables of the respective object types vehicle, car, and truck. All three object types have a scalar attribute for "vehicle ID number." This attribute is called VIN. It is the primary key for each object type.

The vehicle object type contains two attributes of REF type that reference car objects and truck objects respectively in those object tables. (For a given vehicle, only one of these can contain a value: a vehicle cannot be both a car and a truck.)

Car and truck objects can reference vehicle objects via the VIN attribute.