Sometimes referred to as the enhanced entity relationship model, the extended entity relationship model is essentially the entity relationship model that adds more semantic constructs. Within the extended entity relationship model, there is a specialization hierarchy, which depicts the arrangement of high-level entity super types and lower-level entity subtypes. An entity supertype is a generic entity type that contains the common characteristics of entity subtypes. While entity subtypes are a subset of an entity supertype. The entity supertype contains the common characteristic and the subtypes contain the unique characteristic of each entity (Coronel & Morris, 2017).
Withing the extended entity relationship, Inheritance is the property that enables an entity subtype to inherit the attributes and relationships of the supertype. An example of this would be to look at Freshman, Sophomore, Junior, and Senior. They all inherit the STUDENT entity. Entity subtype inherit all relationships in which the supertype entity participates (Coronel & Morris, 2017). The attribute within the supertype entity that determines to which subtype the supertype occurrence is related is known as the subtype discriminator. For the example above, the subtype discriminator is the class type (Coronel & Morris, 2017).
Entity supertypes can have disjoint entity subtypes and Overlapping subtypes. Disjoint subtype, or nonoverlappying subtypes, are subtypes that contain a unique subset of the supertype entity set. While overlapping subtypes are subtypes that contain a nonunique subsets of the supertype entity set. Meaning that each entity instance of the supertype may appear in more than one subtype. The constraint that can be either partial or total is the completeness constraint, which specifies whether each entity supertype occurrence must also be a member of at least one subtype (Coronel & Morris, 2017).
To round off the key components of an EER model, we have specialization and generalization. Specialization is the top down process of identifying lower level, more specific entity subtype from a higher level entity supertype. While generalization is the bottom-up process of identifying a higher level, more generic entity supertype from the lower level entity subtypes (Coronel & Morris, 2017).
References
Coronal, C. & Morris, S. (2017). Database Systems: Design, Implementation, and Management.
Cengage Learning.
