Difference between ER diagram and EER diagram

Question

What is the difference between ERD (Entity relationship diagram) and EERD (enhanced entity relationship diagram)?

Answer 1

The primary features added by the Enhanced ER Model are specialization/generalization and unions. Of these Gen-spec is the one that most often comes up in answers to Stackoverflow questions.

Gen-spec is the counterpart, in ER modeling, of class-subclass in object modeling. An example to illustrate: a motor vehicle could be an auto or a truck. A feature like cargo space pertains to a truck but not to an auto. So there are features that apply to the generalized entity, but the are also features that apply to only one of the specializations.

Very often, people come in here with a question about table design for a case of gen-spec. But they don't recognize it as such, because they were never introduced to gen-spec in their beginning education about databases.

Another way of saying this is that they were taught how to model and implement "has a" relationships, but not how to model and implement "is a" relationships.