Suppose we have an entity set Legal_cars and an entity set License_plate with a relationship called has between them. Which entity set must have mandatory participation in the has relationship? Justify your answer based on the nature of the relationship.

Question

BlackTom AI · Accepted Answer

In this scenario, we are looking at a relationship called has between Legal_cars and License_plate. The real-world interpretation is that every car has a license plate, and every license plate belongs to exactly one car. This implies total participation (mandatory) from both sides of the relationship.

Option: Legal_cars
If only Legal_cars had mandatory participation, it would mean every car must participate in the has relationship (i.e., every car must be related to a license plate), but it would not by itself enforce that every license plate is associated with a car. There could be license plates that do not correspond to any car, which contradicts the intended real-world constraint.

Option: License_plate
If only License_plate had mandatory participation, it would require every license plate to be related to a car, but it would allow cars to exist without a license plate if the Legal_cars side were optional. That would also violate the practical rule that a car should have a license plate.

Option: Both
The most accurate modeling, given the usual real-world rule that a car has exactly one license plate and a license plate belongs to exactly one car, is that participation is mandatory on both sides. This ensures that there are no orphan cars without plates and no orphan plates without cars, aligning with the semantics of the has relationship.

Option: Either
This would be too permissive, permitting either side to be unpaired, which does not reflect the constraint that the has relationship should tightly couple each car with a plate and each plate with a car.

Suppose we have an entity set Legal_cars and an entity set License_plate with a relationship called has between them. Which entity set must have mandatory participation in the has relationship? Justify your answer based on the nature of the relationship.单项选择题

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