Lesson 3 - The Enhanced Entity-Relationship (EER) Model

poperties of superclasses and subclasses

-a member entity of the subclass represents the same real-world entity as some member of the superclass -the subclass member is the same as the entity in the superclass, but in a distinct specific role -when implementing a superclass/subclass relationship, a member of the subclass may be represented as a distinct database object -a distinct record that is related via the key attribute to its superclass -an entity in a subclass must also be a member of the superclass -it is not necessary that every entity in a superclass be a member of some subclass -an entity can be a member of more than one subclass

why class/subclass relationships and specializations?

-certain attributes may apply to some but not all entities of the superclass -a subclass is defined in order to group the entities to which these attributes apply -the members of the subclass may still share the majority of their attributes with the other members of the superclass -some relationship types may be participated in only by entities that are members of the subclass -the relationship between a suoerclass and any one of its subclasses is called a superclass/subclass or class/subclass relationship

completeness constraint

-complpeteness constraint may be total or partial -a total specialization constraint specifies that every entity in the superclass must be a member of some subclass in the specialization -represented by a "double line" connecting the superclass to the circle -a partial specialization allows an entity not to belong to any of the subclasses, using a single line in the EER

specialization

-process of defining a set of subcalsses of an entity type -defined on the basis of some distinguishing characteristic of the entities in the superclass -allows you to do the following: -define a set of subclass of an entity type -establish additional specific attributes with each subclass -establish additional specific relationship types between each subclass and other entity types or other subclasses

generalization

-reverse process of specialization -the generalization process corresponds to a botto-up conceptual synthesis -start with an entity type of subclasses and then define superclasses on the entity type by successive generalization -Generalize into a single superclass -original entity types are special subclasses -generalization supresses the difference among several entity types, identifying their common features, and generalize them into a single supperclass of which the original types are special subclasses -the decision as to which process, generalization or specialization, is more appropriate in a particular situation is often subjective

disjointness constraint

-specifies that the subclass of the specialization must be disjointed -an entity can be a member of at most one of the subclasses of the specialization -in the EER diagram, "d" in the circle stand for disjoint

overlap constraint

-specifies that the subclasses are not constrained to be disjoint -the same (real-world) entity may be a member of more than one subclass of the specialization -overlap is the default constraint and displayed by placing '"o" in the circle

specialization contd

-the process of defining a set of subclass of an entity type (the suerclass of the specialization) -the set of subclasses that form a specialization is defined on the basis of some distinguishing characteristics of the entities in the superclass -a subclass can participate in specific relationship type the same entity type may have several specializations based on different distinguishing characteristics

type inheritance

-the type of the entity is defined by the attributes it possesses and the relationship types which it participates -an entity that is a member of a subclass inherits: -all the attributes of the superclass, as well as -all the relationships in which the superclass participates