Lecture 5: Conceptual Database Design

1. Lecture 5:Conceptual Database Design January 14th, 2003

2. Building an Application with a DBMS • Requirements modeling (conceptual, pictures) • Decide what entities should be part of the application and how they should be linked. • Schema design and implementation • Decide on a set of tables, attributes. • Define the tables in the database system. • Populate database (insert tuples). • Write application programs using the DBMS • way easier now that the data management is taken care of.

3. Database Design • Why do we need it? • Agree on structure of the database before deciding on a particular implementation. • Consider issues such as: • What entities to model • How entities are related • What constraints exist in the domain • How to achieve good designs

4. Database Design Formalisms 1. Object Definition Language (ODL): • Closer in spirit to object-oriented models • I don’t teach it anymore. 2. Entity/Relationship model (E/R): • More relational in nature. • Both can be translated (semi-automatically) to relational schemas • ODL to OO-schema: direct transformation (C++ or Smalltalk based system).

5. 2. Entity / Relationship Diagrams Entities Attributes Relationships between entities Product address buys

6. Keys in E/R Diagrams • Every entity set must have a key name category price Product

7. name category name price makes Company Product stockprice buys employs Person name ssn address

8. makes Company Product 1 a b 2 A= c 3 B= d What is a Relation ? • A mathematical definition: • if A, B are sets, then a relation R is a subset of A x B • A={1,2,3}, B={a,b,c,d}, R = {(1,a), (1,c), (3,b)} - makes is a subset of Product x Company:

9. 1 2 3 1 2 3 1 2 3 a b c d a b c d a b c d Multiplicity of E/R Relations • one-one: • many-one • many-many

10. name category name price makes Company Product stockprice What doesthis say ? buys employs Person name ssn address

11. Product Purchase Store Person Multi-way Relationships How do we model a purchase relationship between buyers, products and stores? Can still model as a mathematical set (how ?)

12. Invoice VideoStore Rental Movie Person Arrows in Multiway Relationships Q: what does the arrow mean ? A: if I know the store, person, invoice, I know the movie too

13. Arrows in Multiway Relationships Q: what do these arrow mean ? A: store, person, invoice determines movie and store, invoice, movie determines person Invoice VideoStore Rental Movie Person

14. Invoice VideoStore Rental Movie Person Arrows in Multiway Relationships Q: how do I say: “invoice determines store” ? A: no good way; best approximation: Q: Why is this incomplete ?

15. Roles in Relationships What if we need an entity set twice in one relationship? Product Purchase Store buyer salesperson Person

16. Attributes on Relationships date Product Purchase Store Person

17. Converting Multi-way Relationships to Binary ProductOf date Product Purchase StoreOf Store BuyerOf Person

18. From E/R Diagramsto Relational Schema • Entity set  relation • Relationship  relation

19. Entity Set to Relation name category price Product Product(name, category, price) name category price gizmo gadgets $19.99

20. Relationships to Relations price name category Start Year name makes Company Product Stock price Makes(product-name, product-category, company-name, year)Product-name Product-Category Company-name Starting-year gizmo gadgets gizmoWorks 1963 (watch out for attribute name conflicts)

21. Relationships to Relations price name category Start Year name makes Company Product Stock price No need for Makes. Modify Product: name category price StartYear companyName gizmo gadgets 19.99 1963 gizmoWorks

22. Multi-way Relationships to Relations address name Product Purchase Store price name Person Purchase( , , ) ssn name

23. 3. Design Principles What’s wrong? Purchase Product Person President Country Person Moral: be faithful!

24. Design Principles:What’s Wrong? date Product Purchase Store Moral: pick the right kind of entities. personAddr personName

25. Design Principles:What’s Wrong? date Dates Product Purchase Store Moral: don’t complicate life more than it already is. Person

26. Modeling Subclasses • The world is inherently hierarchical. Some entities are special cases of others • We need a notion of subclass. • This is supported naturally in object-oriented formalisms. Products Software products Educational products

27. Subclasses in E/R Diagrams name category price Product isa isa Software Product Educational Product platforms Age Group

28. field1 field1 field1 field2 field2 field2 Understanding Subclasses • Think in terms of records: • Product • SoftwareProduct • EducationalProduct field3 field4 field5

29. name category price Product isa isa Software Product Educational Product platforms Age Group Product Subclasses to Relations Sw.Product Ed.Product

30. FurniturePiece Company Person Modeling UnionTypes With Subclasses Say: each piece of furniture is owned either by a person, or by a company

31. Person FurniturePiece Company ownedByPerson ownedByPerson Modeling Union Types with Subclasses Say: each piece of furniture is owned either by a person, or by a company Solution 1. Acceptable, imperfect (What’s wrong ?)

32. Modeling Union Types with Subclasses Solution 2: better, more laborious Company Owner isa isa ownedBy Person FurniturePiece

33. Constraints in E/R Diagrams Finding constraints is part of the modeling process. Commonly used constraints: Keys: social security number uniquely identifies a person. Single-value constraints: a person can have only one father. Referential integrity constraints: if you work for a company, it must exist in the database. Other constraints: peoples’ ages are between 0 and 150.

34. Keys in E/R Diagrams name category Underline: price Product No formal way to specify multiple keys in E/R diagrams Person name ssn address

35. Single Value Constraints makes v. s. makes

36. Referential Integrity Constraints makes Product Company makes Product Company

37. Other Constraints makes <100 Product Company What does this mean ?

38. Weak Entity Sets Entity sets are weak when their key comes from other classes to which they are related. affiliation Team University sport number name

39. Handling Weak Entity Sets affiliation Team University sport number name Convert to a relational schema (in class)