1. Cse 344
May 11th – Entities

2. Administrivia HW6 Due next Wednesday (May 16)
Section 7 slides very helpful
HW7 Out Wednesday
Due May 23rd
HW8 Out May 23rd
Due last day of class, Jun 1
Exam
Graded and on canvas tonight
Back in class or in OH on Monday

3. Database Design
What it is: Starting from scratch, design the database schema: relation, attributes, keys, foreign keys, constraints etc Why it’s hard The database will be in operation for a very long time (years). Updating the schema while in production is very expensive (why?)

4. Database Design Consider issues such as: Several formalisms exists
What entities to model
How entities are related
What constraints exist in the domain
Several formalisms exists
We discuss E/R diagrams
UML, model-driven architecture
Reading: Sec
It is not easy to go from real-world entities to a database schema. One tool to help along the way are E/R diagram.
This is a topic that has been studied for decades – in fact there has been an annual conference just on conceptual modeling!

5. Database Design Process
company
makes
product
name
price
address
Conceptual Model:
Relational Model:
Tables + constraints
And also functional dep.
Normalization:
Eliminates anomalies
We have covered physical schema earlier. Now let’s move on to the upper layers in the design process
Conceptual Schema
Physical storage details
Physical Schema

6. Entity / Relationship Diagrams
Entity set = a class
An entity = an object
Attribute
Relationship
Product
city
Three principal element types
Relationships are connections among two or more entity sets.
Relationships can have attributes too
makes

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

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

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

10. Multiplicity of E/R Relations
one-one:
many-one
many-many 1 2 3 a b c d 1 2 3 a b c d
One-one: student name and ID
Many-one: employees and company
Many-Many: books and authors 1 2 3 a b c d

11. name
CEO
price
address
ssn
buys
makes
employs
Company
Product
What does this say ?
Person

12. Attributes on Relationships
What does this say ?
name
price
name
address
date
Buys
Person
Product

13. Multi-way Relationships
How do we model a purchase relationship between buyers,
products and stores?
date
Purchase
Product
Person
Store
Multi-way relationships are rare in practice
Can still model as a mathematical set (How?)
As a set of triples ⊆ Person × Product × Store

14. Arrows in Multiway Relationships
date
Q: What does the arrow mean ?
Product
Purchase
Store
Person
Arrow pointing to E means that if we select one entity from each of the other entity sets in the relationship, those entities are related to at most one entity in E.
A: Any person buys a given product from at most one store
[Fine print: Arrow pointing to E means that if we select one entity from each of the other entity sets in the relationship, those entities are related to at most one entity in E]

15. Arrows in Multiway Relationships
Q: What does the arrow mean ?
date
Product
Purchase
Store
Person
A: Any person buys a given product from at most one store
AND every store sells to every person at most one product

16. Converting Multi-way Relationships to Binary
ProductOf
date
Product
Purchase
StoreOf
Store
BuyerOf
Person
Arrows go in which direction?

17. Converting Multi-way Relationships to Binary
ProductOf
date
Product
Purchase
StoreOf
Store
BuyerOf
Person
Make sure you understand why!

18. 3. Design Principles What’s wrong? Purchase Product Person President
Country
Design does not reflect reality
Person
Moral: Be faithful to the specifications of the application!

19. Design Principles: What’s Wrong?
date
Product
Purchase
Store
We have redundancy: repeat address of person for each purchase. More next lecture.
Moral: pick the right
kind of entities.
personAddr
personName

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

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

22. Entity Set to Relation Product(prod-ID, category, price) prod-ID
Gizmo55
Camera
99.99
Pokemn19
Toy
29.99

23. N-N Relationships to Relations
prod-ID
cust-ID
date
name
date
Shipment
Shipping-Co
Orders
address
Represent this in relations

24. N-N Relationships to Relations
prod-ID
cust-ID
date
name
date
Shipment
Shipping-Co
Orders
Orders(prod-ID,cust-ID, date)
Shipment(prod-ID,cust-ID, name, date) Shipping-Co(name, address)
address
prod-ID
cust-ID
name
date
Gizmo55
Joe12
UPS
4/10/2011
FEDEX
4/9/2011

25. N-1 Relationships to Relations
prod-ID
cust-ID
date
name
date
Shipment
Shipping-Co
Orders
address
Represent this in relations

26. N-1 Relationships to Relations
prod-ID
cust-ID
date
name
date
Shipment
Shipping-Co
Orders
address
Orders(prod-ID,cust-ID, date1, name, date2) Shipping-Co(name, address)
Remember: no separate relations for many-one relationship

27. Multi-way Relationships to Relations
address
name
Product
Purchase
price
Store
prod-ID
Person
Try this at home!
ssn
name
Purchase(prod-ID, ssn, name)