3.5 Relationships
Entities are often linked by associations or relationships, which are connections or interactions among the entity instances. For example, a student may be related to a class by being enrolled in that class. By abstraction, we can identify the common properties of certain relationships and define a relationship type and a corresponding well-defined relationship set as the collection of relationships of that type. The relationships that satisfy the requirements for membership in the relationship set at any moment are the instances, or members, of the relationship set. As with entities and attributes, the relationship type is part of the intension and the instances are part of the extension of the model. For example, we have ...
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