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Chapter 4

Chapter 4. The enhanced ER model and business rules. Evolution of the E-R Model. Basic E-R Model nearly 25 years old complex data relationships and new database technology have outgrown it in some respects Enhanced E-R Model a response to the shortcomings of the basic E-R model

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Chapter 4

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  1. Chapter 4 The enhanced ER model and business rules

  2. Evolution of the E-R Model • Basic E-R Model • nearly 25 years old • complex data relationships and new database technology have outgrown it in some respects • Enhanced E-R Model • a response to the shortcomings of the basic E-R model • not universally agreed upon in some respects • introduced the supertype/subtype relationship

  3. Supertype/Subtype Relationships • Supertype (example: Employee) • a generic entity that has a relationship with one or more subtypes • Subtype (example: Manager) • a subgrouping of a supertype entity that is meaningful to an organization • shares all attributes of its supertype, but also has unique attributes of its own and/or : • has relationships with other entities distinct from those of other subtypes

  4. Supertype/Subtype Notation

  5. The student example STUDENT UNDERGRAD STUDENT GRAD STUDENT

  6. Two Rules for When to Use Supertype/Subtypes Use this type of relationship when either (or both) of the following are present: 1. When there are attributes that apply to some (but not all) of the instances of an entity type 2. When the instances of a subtype participate in a relationship unique to that subtype

  7. Attribute Inheritance “The property by which subtype entities inherit values of all attributes of the supertype. This important property makes it unnecessary to include supertype attributes redundantly with the subtypes.”

  8. Name City/State/Zip SSN EMPLOYEE Telephone HOURLY EMPLOYEE SALARIED EMPLOYEE CONSULTANT Annual_Salary Stock_Option Contract_Number Billing_Rate Hourly_Rate Supertype/Subtype Example 1

  9. Supertype/subtype relationships in a hospital

  10. Generalization The process of defining a more general entity type from a set of more specialized entity types A “bottom-up” approach Specialization The process of defining one or more subtypes of a general entity based on distinguishing attri-butes or relationships A “top-down” approach Two Processes to Develop Supertype/Subtypes Both approaches can be used together

  11. Completeness Addresses the question of whether an instance of a supertype must also be a member of at least one subtype Disjointness Addresses the question of whether an instance of a supertype may simultaneously be a member of two (or more) subtypes Supertype/Subtype Constraints

  12. Completeness Constraint: Two Possible Rules • Total Specialization Rule (Double-line notation) Specifies that each entity instance of the supertype must be a member of some subtype in the relationship (Example: all STUDENTS are either UNDERGRADUATE or GRADUATE students) • Partial Specialization Rule (Single-line notation) Specifies that an entity instance of the supertype is allowed to not belong to any subtype (Example: FACULTY and STAFF are not the only possible members of the entity EMPLOYEE)

  13. Completeness constraint • Can an entity instance be a member of the supertype but no subtype? • Total specialization • partial specialization

  14. Disjointed constraint • Can an entity instance simultaneously be a member of two or more subtypes? • Disjoint rule • overlap rule

  15. Subtype discriminators • Attribute of the supertype whose value determines to which subtype an instance belongs

  16. What does that look like?

  17. Yet another example….

  18. Supertype/subtype hierarchy • Arrangement of super- and subtypes where each subtype has only one supertype.

  19. Another example...

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