EER

• An entity that is member of a subclass inherits

• All attributes of the entity as a member of the superclass • All relationships of the entity as a member of the superclass

we have four types of specialization/generalization:

• Disjoint, total • Disjoint, partial • Overlapping, total • Overlapping, partial

Disjointness Constraint:

• Specifies that the subclasses of the specialization must be disjoint: • an entity can be a member of at most one of the subclasses of the specialization • Specified by d in EER diagram

If not disjoint, specialization is overlapping:

• that is the same entity may be a member of more than one subclass of the specialization • Specified by o in EER diagram

IS-A relationships

SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, ....

• EER stands for

Enhanced ER or Extended ER

EER Model Concepts

Includes all modeling concepts of basic ER • Additional concepts: • specialization/generalization • categories (UNION types)

Completeness Constraint:

Total specifies that every entity in the superclass must be a member of some subclass in the specialization/generalization • Shown in EER diagrams by a double line

Attribute inheritance

is an entity in a subclass may posses subclass specific attribu tes, as well as those associated with the superclass

Subclass

is an entity type that has a distinct role and is also a member of the superclass

Superclass

is an entity type that includes distinct subclasses that require to be represented in a data model

Generalization is the reverse of the

specialization process

EER diagrams extend ER diagrams to represent these additional subgroupings, called

subclasses or subtypes