Enhanced ER(database)
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  • 1. Lecture 4: Enhanced E-R Model ISOM3260, Spring 2014
  • 2. 2 Where we are now • Database environment – Introduction to database • Database development process – steps to develop a database • Conceptual data modeling – entity-relationship (ER) diagram; enhanced ER • Logical database design – transforming ER diagram into relations; normalization • Physical database design – technical specifications of the database • Database implementation – Structured Query Language (SQL), Advanced SQL • Advanced topics – data and database administration
  • 3. 3 Database development activities during SDLC Top-down analysis
  • 4. 4 Why Enhanced E-R Model? • Business data are now more complex • EER improve flexibility of the basic E-R model to represent business data • EER is the result of extending the original E-R model with new modeling constructs
  • 5. 5 Enhanced E-R Model • Supertype and subtype • Generalization and specialization • Specifying constraints in supertype/subtype relationships • Supertype/subtype hierarchy • Entity clustering
  • 6. 6 Supertypes and Subtypes • Subtype – a subgrouping of the entities in an entity type, that is meaningful to the organization, and which has attributes or relationships that are distinct from those in other subgroupings – at the same time, the subgroupings share common attributes which are associated with the entity type – e.g. GRADUATE STUDENT and UNDERGRADUATE STUDENT  subtypes of STUDENT
  • 7. 7 Supertypes and Subtypes • Supertype – a generic entity type that has a relationship with one or more subtypes – e.g. STUDENT  supertype of GRADUATE STUDENT and UNDERGRADUATE STUDENT • Attribute Inheritance – subtype entities inherit values of all attributes of the supertype – an instance of a subtype is also an instance of the supertype
  • 8. 8 Figure 3-1: Basic notation for supertype/subtype relationships
  • 9. 9 Figure 3-2: Employee supertype with three subtypes All subtypes will have Employee_Number, Employee_Name, Address, and Date_Hired Each subtype will also have its own attributes
  • 10. 10 Supertype/Subtype Relationships • Relationships at the supertype level – indicate that all subtypes will participate in the relationship • Relationships at the subtype level – the instances of a subtype may participate in a relationship unique to that subtype
  • 11. 11 Figure 3-3: Supertype/subtype relationships in a hospital Both outpatients and resident patients are cared for by a responsible physician Only resident patients are assigned to a bed
  • 12. 12 Generalization and Specialization • Two processes to develop supertype/subtype relationships • Generalization – the process of defining a more general entity type from a set of more specialized entity types – bottom-up process • Specialization – the process of defining one or more subtypes of the supertype, and forming supertype/subtype relationships – top-down process
  • 13. 13 Figure 3-4a: Example of generalization Three entity types: CAR, TRUCK, and MOTORCYCLE All these types of vehicles have common attributes
  • 14. 14 Figure 3-4b: Generalization to VEHICLE supertype So we put the shared attributes in a supertype No subtype for motorcycle, since it has no unique attributes
  • 15. 15 Figure 3-5a: Example of specialization Applies only to manufactured parts Applies only to purchased parts Parts may be manufactured internally and/or purchased from suppliers
  • 16. 16 Figure 3-5b: Specialization to MANUFACTURED PART and PURCHASED PART Multivalued attribute “supplier” was replaced by a relationship to another entity Created 2 subtypes
  • 17. 17 Completeness Constraints in Supertype/Subtype Relationships • Completeness Constraint – whether an instance of a supertype must also be a member of at least one subtype • Total Specialization Rule – Yes (double line) • Partial Specialization Rule – No (single line)
  • 18. 18 Figure 3-6: Examples of completeness constraints (a) Total specialization rule A patient must be either an outpatient or a resident patient
  • 19. 19 Figure 3-6b: Partial specialization rule A vehicle could be a car, a truck, or neither
  • 20. 20 Disjointness Constraints in Supertype/Subtype Relationships • Disjointness Constraint – whether an instance of a supertype may simultaneously be a member of two or more subtypes • Disjoint Rule – No (letter ‘d’) – an instance of the supertype can be only ONE of the subtypes • Overlap Rule – Yes (letter ‘o’) – an instance of the supertype can simultaneously be a member of two or more subtypes
  • 21. 21 (a) Disjoint rule Figure 3-7: Examples of disjointness constraints A patient can either be outpatient or resident, but not both, at any given time
  • 22. 22 Figure 3-7b: Overlap rule A part may be both purchased and manufactured
  • 23. 23 Subtype Discriminators • A mechanism to implement supertype/subtype relationships • Subtype Discriminator – an attribute of the supertype whose values determine the target subtype(s) • Disjoint subtype – a simple attribute with alternative values to indicate the possible subtypes • Overlapping subtype – a composite attribute whose subparts pertain to different subtypes – each subpart contains a boolean value (‘Y’ or ‘N’) to indicate whether or not the instance belongs to the associated subtype
  • 24. 24 Figure 3-8: Introducing a subtype discriminator (disjoint rule) A simple attribute with different possible values indicating the subtype
  • 25. 25 Figure 3-9: Subtype discriminator (overlap rule) A composite attribute with sub-attributes indicating “Y” or “N” to determine whether it is of each subtype
  • 26. 26 Supertype/Subtype Hierarchies • Attributes are assigned at the highest logical level in the hierarchy – e.g. SSN applies to all PERSONs – e.g. Date_Hired applies only to EMPLOYEE • Subtypes lower in the hierarchy inherit attributes from their supertypes up to the root (entity type at top of hierarchy) – e.g. an instance of FACULTY has the following attributes: – SSN, Name, Address, Gender, Date_of_Birth (from PERSON) – Date_Hired, Salary (from EMPLOYEE) – Rank (from FACULTY)
  • 27. 27 Figure 3-10: Example of supertype/subtype hierarchy
  • 28. 28 Entity Clusters • EER diagrams are difficult to read when there are too many entities and relationships • Different people may be interested in different types of data • Solution – group entities and relationships into entity clusters • Entity cluster – a set of one or more entity types and associated relationships grouped into a single abstract entity type
  • 29. 29 Figure 3-13a: Possible entity clusters for Pine Valley Furniture Related groups of entities can become clusters
  • 30. 30 Figure 3-13b: EER diagram of entity clusters More readable, isn’t it?
  • 31. 31 Figure: ER model for class scheduling
  • 32. 32 Review Questions • What are subtypes and supertypes? • What are generalization and specialization? • What are the total and partial specialization rules? • What are the disjoint and overlap rules? • What are subtype discriminators? • What are supertype/subtype hierarchies? • What are entity clusters?