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When I diagram classes or MER, the cardinality should be defined.
As an example: A doctor consults from 0 to n patients; a patient is consulted by 0 or 1 doctor. Why the 0?
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It is always very difficult to talk about generic and abstract cases like this. The concrete case may be different.
As far as I understand there’s no need for the doctor to have patients. So it could be 0 to N. Of course you can establish a business rule that requires that he have any related patients. And since you are setting a minimum of 1, maybe you can define a minimum greater than 1. For me this rule does not make much sense, but it is as I said. Every business with its rule.
Even the patient doesn’t need to be tied to a doctor. In this case there is a greater chance of having a doctor in certain situations. I would not make this demand either, I see no reason to have it, but if whoever is asking for it says that it justifiably needs some extra restriction, there is no reason not to.
What I would even think is if the patient can’t be tied up to N doctors. Why is there exclusivity? It may have, but I find it unlikely in a hospital or clinic. Even if the clinic only has a doctor, this is a circumstance.
Of course if the system is controlling the doctor’s allocation these rules may be different. But I don’t know if they would be. Nothing prevents the doctor from attending several patients simultaneously as well as a patient may be receiving attention from several doctors, perhaps forming a joint.
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A doctor may not have consulted any patients yet. The patient is strange, because if he is a patient, it is automatically assumed that he has consulted at least once, in this case it would be 1:n doctors.
– AlfredBaudisch
I imagine the fact that the patient has a record does not imply that he has a doctor.
– Rodrigo Rigotti
Zero means that from that side there may be no record. The reason you have to ask for whom you have defined that it is so.
– Caffé
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