EER Model
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All about Enhanced-Entity-Relationship (EER) Model , created by Rahul MUkherjee

All about Enhanced-Entity-Relationship (EER) Model , created by Rahul MUkherjee

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EER Model Presentation Transcript

  • 1. Enhanced-ER (EER) Model Concepts
    ♦ The EER model introduce the additional concepts of subclasses, superclasses, specialization generalization, attribute inheritance. The resulting model is called the enhanced-ER or Extended ER model. It is used to model applications more completely and accurately if needed. It includes some object-oriented concepts, such as inheritance .
    14
  • 2. Subclasses and Superclasses
    • An entity type may have additional meaningful subgroupings of its entities
    • 3. Example: EMPLOYEE may be further grouped into SECRETARY, ENGINEER, MANAGER, TECHNICIAN, SALARIED_EMPLOYEE, HOURLY_EMPLOYEE,…
    • 4. Each of these groupings is a subset of EMPLOYEE entities
    • 5. Each is called a subclass of EMPLOYEE
    • 6. EMPLOYEE is the superclass for each of these subclasses
    14
  • 7. Subclasses and Superclasses
    • These are called superclass/subclass relationships.
    • 8. Example: EMPLOYEE/SECRETARY, EMPLOYEE/TECHNICIAN
    • 9. These are also called IS-A relationships (SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, …).
    • 10. To show class/subclass relationships, use
    U
    14
  • 11. Subclasses and Superclasses
    14
  • 12. Attribute Inheritance in Superclass/Subclass Relationships
    • An entity that is member of a subclass inherits all attributes of the entity as a member of the superclass
    • 13. It also inherits all relationships
    14
  • 14. Specialization
    • Specialization constructs the lower level entity sets that are a subset of a higher level entity set.
    • 15. Is the process of defining a set of subclasses of a superclass
    • 16. The set of subclasses is based upon some distinguishing characteristics of the entities in the superclass
    • 17. Example: {SECRETARY, ENGINEER, TECHNICIAN} is a specialization of EMPLOYEE based upon job type.
    • 18. May have several specializations of the same superclass
    14
  • 19. Specialization
    • Example: Another specialization of EMPLOYEE based in method of pay is {SALARIED_EMPLOYEE, HOURLY_EMPLOYEE}.
    • 20. Superclass/subclass relationships and specialization can be diagrammatically represented in EER diagrams
    • 21. Attributes of a subclass are called specific attributes. For example, TypingSpeed of SECRETARY
    14
  • 22. Example of Specialization
    14
  • 23. Generalization
    • Generalization is the result of computing the union of two or more entity sets to produce a higher-level entity set. It represents the containment relationship that exists between the higher-level entity set and one or more lower-level entity sets.
    • 24. A bottom-up design process – combine a number of entity sets that share the same features into a higher-level entity set.
    14
  • 25. Generalization
    • The reverse of the specialization process
    • 26. Several classes with common features are generalized into a superclass; original classes become its subclasses
    • 27. Example: CAR, TRUCK generalized into VEHICLE; both CAR, TRUCK become subclasses of the superclass VEHICLE.
    • 28. We can view {CAR, TRUCK} as a specialization of VEHICLE
    • 29. Alternatively, we can view VEHICLE as a generalization of CAR and TRUCK
    14
  • 30. GENERALIZATION AND SPECIALIZATION
    sid
    student
    name
    is A
    Specialization
    Generalization
    graduate
    Undergrad
    14
  • 31. Constraints
    • Disjointness Constraint:
    • 32. Specifies that the subclasses of the specialization must be disjointed (an entity can be a member of at most one of the subclasses of the specialization)
    • 33. Specified by d in EER diagram
    • 34. If not disjointed, overlap; that is the same entity may be a member of more than one subclass of the specialization
    • 35. Specified by o in EER diagram
    14
  • 36. Constraints
    • Completeness Constraint:
    • 37. Total specifies that every entity in the superclass must be a member of some subclass in the specialization/ generalization
    • 38. Shown in EER diagrams by a double line
    • 39. Partial specialization – each entity of a superclass does not have to belong to some subclass of a specialization
    • 40. Shown in EER diagrams by a single line
    14
  • 41. Constraints and example
    14