Enhanced Entity–Relationship Model (EER Model)

 The model that has resulted from the extending original ER model with new modeling construct is called enhanced entity relationship model. The most important new modeling construct in EER are super type and sub type.

The basic concepts of ER modeling are not powerful enough for some complex applications. Hence some additional semantic modeling concepts are required, which are being provided by Enhanced ER model. The Enhanced ER model is the extension of the original ER model with new modeling constructs. The new modeling constructs introduced in the EER model are supertype (superclass)/ subtype (subclass) relationships. The supertype allows us to model general entity type whereas the subtype allows us to model specialized entity types.

1. Supertype: A generic entity that have a relation with one or more subtypes .

2. Subtype: A sub grouping of the entities type which has attributes that are distinct from those in other sub-grouping  

Enhanced ER model = ER model + hierarchical relationships.

EER modeling is especially useful when the domain being modeled is object-oriented in nature and the use of inheritance reduces the complexity of the design. The extended ER model extends the ER model to allow various types of abstraction to be included and to express constraints more clearly.

In the above notation the supertype is connect with a line of circle, which turns a line to each subtype that has been defined connect. The 'U' - shape symbol and each line connecting a subtype circle indicates that the subtype is a subset of supertype. It also indicates the direction of subtype and supertype relationship. 

Attributes that are shared by all entities are associated with supertype. Attributes that are unique to a particular subtype are associated with that subtype.