To be considered: The bolded terms that appear across the requirement I laid out bellow are the ones included in the entity set (Vehicle) I created. I also specified the attributes nature/kind/type.
Consider a database for department of motor vehicles (the “dreaded” DMV). In this database, each vehicle has a make and a model, and a unique vehicle identification number(1_VEH_NUM). For the purposes of DMV, a vehicle can be either a two-wheel vehicle(2-wheel-veh) or a (4-wheel-veh)four-wheel vehicle(or neither).
DMV further defines cars(composite attribute) and race cars(composite attribute) as special types of four-wheel vehicles. It is possible that a race car is also a (regular) car (and vice-versa) - i.e., this vehicle is licensed to be driven on public roads and as well as on a race track.DMV does not consider four-wheel vehicles that are not (regular) cars or race cars.
DMV keeps track of licenses plate numbers(component attributes) for all two-wheel vehicles and four-wheel vehicles (but not for race cars). For race cars, a safety index(component attributes) (a value between 1 and 10) and a class(component attributes)(e.g., WRC (Rally), Formula 1, etc) needs to be stored in the database. DMV also stores the maximum number of persons(component attributes; maximum_persons) that are allowed to be carried in each four-wheeled vehicle.
Try to establish a E-R Diagram that accurately depicts the presented scenario.
I considered the Vehicles an entity set which encompasses the following attributes: 1_VEH_NUM(primary key) make model
The entity set Vehicles participates to the relationship set through 2-wheel-veh and 4-wheel-veh which establish the connection with Vehicle_Types entity set. The aforementioned entity set contains a primary-key composite attribute - type and its related neighbor maximum_persons(this is questionably as I don't really know whether a primary could be even a composite attribute or not).
The cardinality of 4-wheel-veh is total on both sides as both type of 4 wheel vehicles could be either race or regular. As for the 2-wheel-veh relationship I perceived it as having a one-to-one cardinality especially because there's a single way to uniquely identify these ones - license plate.