If the population of an object type Amust always be a subset of the population of
another object type B,then Ais said to be a subtypeof B,and Bis said to be a super-
type of A. In ORM, this is depicted visually by a solid arrow running from the subtype
to the supertype. For example, Figure 5–17 shows a fragment of a model used for a hos-
pital information system. In this model, MalePatient and FemalePatient are each sub-
types of Patient.
The main reason for introducing a subtype is to declare that specific roles are played
only by that type. For example: prostate status may be recorded only for male patients;
pregnancy counts and ...
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