1. Lecture 06 Data Modeling: E/R Diagrams
Monday, October 10, 2005

2. Outline E/R diagrams (Chapter 2)
From E/R diagrams to relations (3.2, 3.3)
The relational data model: 3.1
Next time: Functional dependencies: 3.4

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

4. From E/R Diagrams to Relational Schema
Entity set  relation
Relationship  relation

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

6. 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
(watch out for attribute name conflicts)

7. 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 gizmoWorks

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

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

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

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

12. Modeling Subclasses Some objects in a class may be special
define a new class
better: define a subclass
Products
Software
products
Educational
products
So --- we define subclasses in E/R

13. Subclasses name category price Product isa isa Software Product
Educational Product
platforms
Age Group

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

15. Subclasses to Relations
Product
Name
Price
Category
Gizmo 99
gadget
Camera 49
photo
Toy 39
Product
name
category
price
isa
Educational Product
Software Product
Age Group
platforms
Sw.Product
Name
platforms
Gizmo
unix
Ed.Product
Name
Age Group
Gizmo
todler
Toy
retired

16. Difference between ODL and E/R inheritance
ODL: classes are disjoint (same for Java, C++)
Product p1 p2 p3
sp1
sp2
ep1
ep2
ep3
EducationalProduct
SoftwareProduct

17. Difference between ODL and E/R inheritance
E/R: entity sets overlap
Product p1 p2 p3
sp1
sp2
ep1
ep2
ep3
EducationalProduct
SoftwareProduct

18. No need for multiple inheritance in E/R
we have three entity sets, but four different kinds of objects
Still needed if we want extra attributes
Product p1 p2 p3
sp1
sp2
ep1
ep2
ep3
EducationalProduct
SoftwareProduct
esp2
esp1

19. Modeling Subclass Structure
Product
ageGroup
topic
Platforms
required memory
isa
isa
Educational
Product
Software
Product
isa
isa
Educ-software
Product
Educational-method

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

21. 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 ?)
FurniturePiece
Person
Company
ownedByPerson

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

23. 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.

24. 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

25. Single Value Constraints
makes
v. s.
makes

26. Referential Integrity Constraints
makes
Product
Company
Each product made by at most one company.
Some products made by no company
makes
Product
Company
Each product made by exactly one company.

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

28. 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
Notice: we encountered this when converting multiway relationships to binary relationships (last lecture)

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

30. The Relational Data Model
Schema
Physical
storage
Data
Modeling
Have seen this in SQL
Complex
file organization
and index
structures.
E/R diagrams
Tables:
column names: attributes
rows: tuples
Have seen this too
Discuss next

31. Terminology Table name or relation name Attribute names Products:
Name Price Category Manufacturer
gizmo $ gadgets GizmoWorks
Power gizmo $ gadgets GizmoWorks
SingleTouch $ photography Canon
MultiTouch $ household Hitachi
We know this already…
Tuples or rows or records

32. Schemas Relational Schema: Relation name plus attribute names
E.g. Product(Name, Price, Category, Manufacturer)
Database Schema
Set of relational schemas
E.g. Product(Name, Price, Category, Manufacturer),
Company(Name, Address, Phone) …
We know this too…

33. Instances
Relational schema = R(A1,…,Ak): Instance = relation with k attributes (of “type” R)
Database schema = R1(…), R2(…), …, Rn(…) Instance = n relations, of types R1, R2, ..., Rn

34. Example Schema: Product(Name, Price, Category, Manufacturer) Instance:
gizmo $ gadgets GizmoWorks
Power gizmo $ gadgets GizmoWorks
SingleTouch $ photography Canon
MultiTouch $ household Hitachi

35. First Normal Form (1NF)
A database schema is in First Normal Form if all tables are flat
Student
Student
Name
GPA
Alice
3.8
Bob
3.7
Carol
3.9
Name
GPA
Courses
Alice
3.8
Bob
3.7
Carol
3.9
Math DB OS
Takes
Course
Student
Course
Alice
Math
Carol DB
Bob OS
Course
Math DB OS DB OS
May need to add keys
Math OS

36. Normal Forms 1NF = rather trivial 3NF, BCNF = very subtle; next time
Other normal forms = in the book