2. 1 CONCEPTUAL SCHEMA DESIGN PROCESS Information Processing and Technology, 2001 Yr 12 IPT

3. Design Cycle 1.State Elementary Facts 3.Entity types that should be combined and derivation. 5.Add other constraints: value, cardinality, frequency and mandatory. 6.Perform Population and Final Checks. 4.Add Uniqueness Constraints. 2.Draw Conceptual Schema Diagram

4. Universe of Discourse (UoD) The UoD is the domain of information being discussed by the client and database designer.

5. Step 1. State Elementary Facts This step involves expressing information relevant to the UoD as elementary facts (EF). EF are simple unambiguous sentences that make statements about the UoD. EF are irreducible. They cannot be broken down into smaller parts and still convey the same information. EF do not use terms such as and, or, not, if, all, some or most.

6. Student (first name) Mary studies / is studied by Subject (name) IPT Reference Scheme Values = Mary, IPT Entities = Student, Subject Ref Modes = first name, name Role = studies / is studied by

7. Arity §The arity of an elementary sentence is the number of entities that associate or play roles. Ternary : Mary received a B for English

8. Step 2. Draw Conceptual Schema Diagram §The Conceptual Schema Diagrams are a graphical representation of the elementary facts obtained from the first step. Entity types are represented by ellipses: Roles are represented by rectangles:

9. Step 3. Entity types that can be combined and derived §Entity types should be combined l to save space l to ensure only a minimum of objects is used §However: l no information should be lost during this stage l the words used in the roles are very important

10. Combining... Item (name) Profit ($) Retail Price ($) Wholesale Price ($)

11. …Combined Item (name) Has profit of Price ($) Has Wholesale Price of Has retail price of Notice how the wording of the roles becomes very important.

12. Derivation... Item (name) Has profit of Price ($) Has Wholesale Price of Has retail price of *

13. Step 4. Add Uniqueness Constraints §Uniqueness constraints are specified in a conceptual schema to avoid data redundancy. §The relations are: l one to one l one to many l many to one l many to many

14. One to one §Does each friend have only one phone number? §Does each phone number have only one friend? Phone (number) Friend (name) has One friend has one phone number Yes

15. One to many §Does each party have only one minister? §Does each minister have only one party? Minister (minister) Party (party) has One Party has many ministers Yes No

16. Many to one §Does each student have only one house? §Does each house have only one student? House (house) Student (name) has Many students have one house Yes No

17. Many to many §Does each teacher have only one subject? §Does each subject have only one teacher? Subject (subject) Teacher (teacher) has Many teachers have many subjects. No

18. Step 5. Add Other Constraints Mandatory, Value, Cardinality, Frequency §Mandatory role constraints indicate which roles must be played by all values of an entity type. Roles without mandatory constraints are optional.

19. CS Diagram … Rock Star (name) Birth Year (year) Death Year (year) has 1. Does every Rock Star have a Birth Year? 2. Does every Birth Year have a Rock Star? 3. Does every Rock Star have a Death year? 4. Does every Death Year have a Rock Star? Yes No

20. Student (name) Value Constraints §Value Constraints are a listing of allowed values for an entity type §They are used to limit instances for that object to members of a certain set. Gender (letter) {M,F}

21. Grade (letter) {A..E} Cardinality Constraints §Cardinality constraints are similar to Value constraints but limit entities to a range of values. Student (name)

22. Frequency Constraints §Frequency Constraints define the number of times a particular role can be played. §Thus ample storage space can be allocated. <7 Student (name) Subject (subject)

23. 6. Perform Population and Final Checks. §The final Step of the CS Design Phase. §This step must check that the schema is: l internally consistent (no contradicting constraints) l externally consistent (population check) l free of redundancy (undetected derivations) l complete (fulfils original requirements and represents the UoD)