Notes > Database Systems > Enhanced ER Modelling (EER)
Enhanced ER Modelling (EER) allows for more complex concepts such as specialisation and generalisation which were not in the original ER Model. The EER model added the ability to represent Entity Supertypes and Subtypes as well as Attribute inheritance.
Supertypes and subtypes are used when there exist entities which share common properties. The entity supertype contains the shared properties of all the subtypes. An entity subtype has a more specific role and belongs to a supertype. The entity subtype inherits the properties of the supertype.
An example of supertypes and subtypes can be found within various contexts. In a library system, there may exist the Item entity supertype with subtypes being Books, DVDs, or Magazines etc... Also, within a business, Staff may be divided into Managers, Secretaries and Sales Representatives. Here, Staff is the supertype. The entity subtypes will inherit the common attributes from the Staff supertype (e.g. ID, Name, Address). A Sales Representative subtype will therefore have special attributes unique to a sales person (e.g. Bonus, Sales Area) as well as the general attributes associated with the Staff supertype.
Depending on business or system constraints, an entity may belong to multiple subtypes. For example, a member of Staff could be a Manager as well as a Sales Representative. In some cases, belonging to a subtype may not be mandatory. This means that a member of Staff can exist with no specialised role. They will just have the properties associated with the Staff supertype.
The following image shows how two subtypes inherit from a superclass.
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