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

2. Design Cycle 1.State Elementary Facts 2. 3. 4. 6. 5. Draw Conceptual Schema Diagram Entity Types that should be combined and derivation

3. 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

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

5. …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.

6. Derivation §Some fact types can be derived from other fact types stored in the data base. §They are not stored to reduce space needed. Calculations are performed at run time to calculate the derivations. §Example: Retail price – Wholesale price = Profit. The derived field is shown on the CSD with an asterisk (*) beside the role boxes to represent that it is not part of the physical database.

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

8. Summary §Some entity types can be combined to save space in the CS Diagram. §Some entities can be derived from already existing entity types and their roles.