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