The EE-R ModelEdit
The Extended Entity-Relationship Model is a more complex and high-level model that extends an E-R diagram to include more types of abstraction, and to more clearly express constraints. All of the concepts contained within an E-R diagram are included in the EE-R model, along with additional concepts that cover more semantic information. These additional concepts include generalization/specialization, union, inheritance, and subclass/superclass.
Generalization and SpecializationEdit
You can think of generalization as developing a diagram from the bottom up, whereas you can think of specialization as taking a top down approach to development. These two processes are used to incorporate more meaning into EE-R models.
Specialization corresponds to the notion of subclasses and class inheritance. As represented by class hierarchies, specialization usually involves a superclass that is being specialized into subclasses. The relationship is connected by a specialization circle, which acts as symbol to show the specialization. The specialization circle is also referred to as an isa relationship.
Generalization works in the opposite direction of specialization. By recognizing two or more entities that have common properties in the class hierarchy, generalization creates a common superclass for them. For example, the entities CAR and TRUCK have similar properties and attributes, so they could be generalized into a superclass of AUTOMOBILE.
A union (also known as a category) is a collection of superclasses that acts as a union between objects of different entity types. Instances of the subclass will inherit the attributes of only one of the superclasses, depending on the union that it belongs to.