Extending ER Diagrams (13) CPSC 356 Database Ellen Walker Hiram College (Includes figures from Database Systems: A Practical Approach to Design, Information and Management, by Connolly and Begg, © Addison Wesley 2010)

Basic ER Diagram Cannot Capture Specialization/ Generalization Examples –Doctors and Nurses are Medical Staff –Staff includes Medical staff, Secretarial staff, Housekeeping staff, etc. Relationships might include Doctors, Medical Staff, and/or Staff ER diagram cannot represent the fact that Staff > Medical staff > doctors EER diagram can!

Entity Type Hierarchies An entity type E1 is a subclass of the entity type E2 when –Every entity instance of E1 is also an instance of E2 –Every attribute in E2 is also in E1 The opposite of a subclass is a superclass Subclasses are related to superclasses through the IS-A relationship

When to Use? When you have an entity that needs to have extra / different attributes “sometimes” When you have two entities that overlap on many or most of their attributes

Subclass / Superclass Example Doctor is a subclass of medical-staff Staff is a superclass of medical-staff Attributes –Staff - name, addr, salary –Med-staff - pager # –Doctor - license # Staff Medical-StaffAdmin-Staff DoctorNurse

Attribute Inheritance Each entity type inherits all attributes of its superclass(es) –Staff has name, address, salary –Medical staff has {n,a,s} + beeper-number –Doctor has {n, a, s, b#} + license # In EER diagram, inherited attributes only appear once (in the highest superclass)

Generalizing Type Hierarchies Multiple superclasses for the same subclass –A student-employee is both a student and an employee Overlapping subclasses –Subclasses of person are student and employee –There is at least one individual who is both a student and an employee Non-overlapping subclasses are called “disjoint”

Specialization of Staff (13.2)

Extended ER (EER) Diagram IS-A relationships Notation for disjoint (vs. overlapping) subclasses –Disjoint = “or”, overlapping = “and” Notation for “covering” –Can someone be staff without being Medical or administrative? –If so, “optional”; if not “mandatory”

More Staff Types (13.3)

Additional Semantic Relationships Aggregation and Composition –Collecting together related entities that represent ‘parts’ of a ‘whole’ –Aggregation (non-exclusive subpart): parts can change parents –Composition (exclusive subpart): single parent, very strong association - parts make no sense without parent –Examples: Branch (whole) has Staff (part)Aggregation Newspaper (whole) contains Story (part) Composition EER representation is diamond with arrow from “part” –Open for aggregation, closed for composition –(see 13.9, 13.10)