1. Example
BigHit Video Inc. wants to create an information system for online sales of movies in both DVD and videotape format. People will be allowed to register as customers of the online site and to update their stored information. Information must be maintained about customers’ shipping addresses, addresses and credit cards. In a single sale, customers will be allowed to purchase any quantity of videos. The items in a single sale will be shipped to a single address and will have a single credit card charge.
A customer will be provided with a virtual shopping cart to store items to be purchased. As each item is selected, it is added to the shopping cart. When the customer finishes shopping, he will be directed to a checkout area where payment and shipping information is entered. Once the sale is complete, the customer will be sent a receipt by .

2. Example
Entities: Customer (key: account , last name, first name, address, , credit card, password) Movie (key: id, title, genre, length) Sale (key: id, cost, date, credit card) Shopping Cart (key: id, date)

3. Example Relationship Type Entity Class Cardinality Ratio Attibutes
Purchases
Customer
Sale
one-to-many
Includes
Movie
many-to-many
quantity
Selects
ShoppingCart

4. Data Modeling with E-R Diagram
An Entity-Relationship (E-R) model is a high-level conceptual model
E-R models are best described using graphical diagrams
This model provides designers with an accurate and useful model that avoids confusion and is understandable by users

5. E-R model An E-R model includes
Entity classes
Attributes of each class
Relationship types between classes
Constraints
Types of attributes
Designation of key attributes
Cardinalities of relationship types
An E-R Model is typically represented graphically
using an E-R diagram

6. E-R model
E-R models cannot describe everything in the real-world application
Not all information about the attributes appears in the E-R diagram. Most of the information is maintained in the data dictionary
The process of designing the E-R diagram is iterative

7. E-R Diagrams: Entity Classes
Entity classes are represented by rectangles with the name inside
Attributes are represented by ovals.
key attributes are underlined
Multi-valued attributes are double-lined
Derived attributes are dashed

8. Example of an entity class

9. Example

10. E-R diagrams: Relationships
A relationship is represented by a rhombus (diamond) with the name inside
The name of a relationship is a verb describing it
Additional symbols of a relationship:
Cardinality
Optional/mandatory participation
Attributes of a relationship

11. Example of a relationship

12. Example

13. Modeling relations as entity classes

14. Modeling relations as entity classes
A relationship can be modeled as an entity class

15. Modeling relations as entity classes
Note cardinalities and participation constraints
A Rental entity cannot exist without being related to both a customer and a video
Problem occurs because Rental has no key attribute

16. Weak and Strong entities
A weak entity class is
An entity class with no key of its own
An entity class whose entities cannot exist without being related to other entities
An identifying relationship type is
A relationship type that determines the keys of the weak entities

17. Weak and Strong entities

18. More on Weak Entities and Relations
An entity that is identified by its relationship to other entities is called a weak entity and its class a weak entity class
Ssn
Name
EMPLOYEE
111111 1
DEPENDENTS_OF N
DEPENDENT
Name
Relation
Bdate

19. More on Weak Entities and Relations
The stronger entity class is called the identifying or owner entity class
The relationship that relates the weak entity class to its owner is called: identifying relationship
A partial key of a weak entity is a set of attributes that uniquely identifies weak entities related to the same owner entity

20. Example
Entities: Customer (key: account , last name, first name, address, , credit card, password) Movie (key: id, title, genre, length) Sale (key: id, cost, date, credit card) Shopping Cart (key: id, date)

21. Example Relationship Type Entity Class Cardinality Ratio Attibutes
Purchases
Customer
Sale
one-to-many
Includes
Movie
many-to-many
quantity
Selects
ShoppingCart