1. 1 Chapter 4 Enhanced E-R Model

2. 2 Supertypes and Subtypes Subtype: A subgrouping of the entities in an entity type which has attributes that are distinct from those in other subgroupings. Supertype: An entity type whose subtypes share common attributes. Attributes that are shared by all entities (including the identifier) are associated with the supertype. (Fig. 4-1, 2)

3. 3 Basic notation for supertype/subtype relationships

4. 4 Employee supertype with three subtypes

5. 5 Attribute Inheritance Subtype entities inherit values of all attributes of the supertype. An occurrence of a subtype is also an occurrence of the supertype.

6. 6 Use of Supertype/Subtype There are attributes that apply to some (but not all) of the instances of an entity type. The instances of a subtype participate in a relationship unique to that subtype. (see Fig. 4-3.)

7. 7 Supertype/subtype relationships in a hospital

8. 8 Generalization and Specialization Generalization: The process of defining a more general entity type from a set of more specialized entity types. (Fig. 4-4) Specialization: The process of defining one or more subtypes of the supertype, and forming supertype/subtype relationships. (Fig. 4-5)

9. 9 Example of generalization (a) Three entity types: CAR, TRUCK, and MOTORCYCLE

10. 10 (b) Generalization to VEHICLE supertype

11. 11 Example of specialization (a) Entity type PART

12. 12 (b) Specialization to MANUFACTURED PART and PURCHASED PART

13. 13 Constraints in Supertype/ Subtype Relationships Completeness Constraints: Whether an instance of a supertype must also be a member of at least one subtype. –Total Specialization Rule: Yes (Fig. 4-6a - note the double line convention) –Partial Specialization Rule: No (Fig. 4-6b)

14. 14 Examples of completeness constraints (a) Total specialization rule

15. 15 (b) Partial specialization rule

16. 16 Constraints in Supertype/ Subtype Relationships Disjointness Constraints: Whether an instance of a supertype may simultaneously be a member of two (or more) subtypes. –Disjoint Rule: No –Overlap Rule: Yes (Fig. 4-7)

17. 17 (a) Disjoint rule Examples of disjointness constraints

18. 18 (b) Overlap rule

19. 19 Constraints in Supertype/ Subtype Relationships Subtype Discriminators: An attribute of the supertype whose values determine the target subtype(s). –Disjoint (Fig. 4-8) –Overlapping (Fig. 4-9) Summary Example (Fig. 4-10)

20. 20 Introducing a subtype discriminator (disjoint rule)

21. 21 Subtype discriminator (overlap rule)

22. 22 Example of supertype/subtype hierarchy

23. 23 Classification of business rules

24. 24 Data model segment for class scheduling

25. 25 Sample Entity Definitions 1.Name: FACULTY Type: Regular Definition: a university employee who is academically qualified to teach courses and perform other designated duties. Identifier: Faculty_ID 2.Name: COURSE Type: Regular Definition: a module of instruction in a particular subject area Identifier: Course_ID 3.Name: SECTION Type: Weak Definition: an offering of a course during a particular semester Identifier: Section_ID (partial only)

26. 26 Sample Attribute Definitions 1.Name: Faculty_ID Domain: Person_IDs Definition: unique identifier of a faculty member Null: no 2.Name: Faculty_Name Domain: Person_Names Definition: name of a faculty member Null: no 3.Name: Course_ID Type: Course_ID Definition: unique identifier of a course Null: no

27. 27 Sample Relationship Definitions 1.Name: Is_qualified Type: Binary M:N Description: associates each faculty member with course(s) he or she is qualified to teach based on academic preparation and experience Constraints: each faculty must be qualified to teach at least one course Attributes: Date_Qualified 2.Name: Is_scheduled Type: Binary M:N Description: associates each course with the sections that have been scheduled for that course. Constraints: identifying relationship Attributes: none

28. 28 Sample Domain Constraints 1.Name: Person_IDs Data Type: Character Allowable Character: digits 2.Name: Person_Names Data Type: Character Max Length: 30 3.Name: Course_IDs Data Type: Character Format: ssss nnnn ssss = subject nnnn = course number

29. 29 Relationship between attributes and domains

30. 30 Representing a business rule (a) Simple E-R diagram segment

31. 31 (b) Adding symbol to enforce business rule

32. 32 Business Rule 1: For a faculty member to be assigned to teach a section of a course, the faculty member must be qualified to teach the course for which that section is scheduled.

33. 33 Business Rule 2: For a faculty member to be assigned to teach a section of a course, the faculty member must not be assigned to teach a total of more than three course sections.