Enhanced-ER (EER) Model Concepts<br />♦ The EER model introduce the additional concepts of subclasses, superclasses, speci...
Subclasses and Superclasses<br /><ul><li>An entity type may have additional meaningful subgroupings of its entities
Example: EMPLOYEE may be further grouped into SECRETARY, ENGINEER, MANAGER, TECHNICIAN, SALARIED_EMPLOYEE, HOURLY_EMPLOYEE,…
Each of these groupings is a subset of EMPLOYEE entities
Each is called a subclass of EMPLOYEE
EMPLOYEE is the superclass for each of these subclasses </li></ul>14<br />
Subclasses and Superclasses<br /><ul><li>These are called superclass/subclass relationships.
Example: EMPLOYEE/SECRETARY, EMPLOYEE/TECHNICIAN
These are also called IS-A relationships (SECRETARY IS-A EMPLOYEE, TECHNICIAN IS-A EMPLOYEE, …).
To show class/subclass relationships, use</li></ul>U<br />14<br />
Subclasses and Superclasses<br />14<br />
Attribute Inheritance in Superclass/Subclass Relationships <br /><ul><li>An entity that is member of a subclass inherits a...
It also inherits all relationships </li></ul>14<br />
Specialization<br /><ul><li>Specialization constructs the lower level entity sets that are a subset of a higher level enti...
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EER Model

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

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EER Model

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

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