If all rooms permitted only two roommates, the relationship could be 1:1, but probably some suites allow for
three, four, or more roommates, so the relationship between student and roommates will be N:M, and we will call
it ???rooms-with???. The minimum cardinality on either side can be 0, if we have some students who will room alone.
Finally, some entity types can represent subclasses and superclasses. For instance, students may be either
undergraduate or graduate students. We would model ???student??? as the superclass, and we would model ???undergraduate???
and ???graduate??? as subclasses. Attributes of student would include those attributes relevant to all students,
such as name, address, etc. Attributes of undergraduate entities would include those attributes relevant only to
undergraduates, such as student life advisor (assuming graduate students have no such advisor assigned).
Figure 8-2 illustrates weak and ID-dependent entities, multivalued attributes, and superclass and subclass
entities.
In Fig. 8-2, the Dependents table is an id-dependent weak entity. The identifier for that table includes the
key for the related strong entity Faculty, plus the name of the dependent (spouse??™s name, child??™s name, etc.
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