A sub-type of an entity class is the definition of a subset of the objects that the super-type entity class de­scribes. Any attributes or relationships that are properties of a super-type are inherited by each sub-type. Any attributes or relationships of a particular sub-type are not inherited by any other sub-type, nor are they properties of the super-type.

Both UML and some versions of Information Engineering notations show sub-types as boxes next to their super-type boxes. This can take up a lot of space on a diagram. Moreover, if the hierarchy is relatively deep, it is not easy to see that an attribute of—or a relationship to—a super-type is in fact also an attribute of—or a relationship to—each of its sub-types, ...

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