Data Modeling with ERD
BUS 782
Entities
• An entity is a person, place, object, event, or
concept in the user environment about which
the organization wishes to maintain data.
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–
–
–
–
Person: Employee, Student, patient
Place: Warehouse, Store
Object: Product, Machine.
Event: Registration, Sale, Renewal
Concept: Account, Course
• Physical existence:
• Customer, student, product, etc.
• Conceptual existence:
• Bank accounts, sale
Entity Type
• A collection of entities that share common
properties or characteristics.
• An entity type represents a collection of
entities.
• A business environment may involve many
entity types.
– University: Faculty, Student, Course
– Department, Employee, Dependent
– Sales person, Customer, Order
Relationship
• Relationship: Interaction between entity types.
– Faculty teach Course, Faculty advise Student
– Customer open Account, Customer purchase
Product.
• Binary relationship: A relationship involves two
entity types.
• Three types of binary relationship:
– 1:1, 1:M, M:M
Entity-Relationship Diagram
• ER modeling begins by identifying the
entities and relationships between entities
that must be represented in the model.
• In an ERD, an entity type is represented
by a rectangle labeled with a singular
name.
• A relationship has a verb phrase name:
– Faculty teach Course, Faculty advise Student
M:M Relationship
Boy
Girl
Peter
Mary
Paul
Linda
John
Nancy
Woody
Mia
Alan
Pia
A boy may date 0, 1, or many girls.
A girl may date 0, 1, or many boys.
Note: “Many boys date many girls” is not a correct
interpretation.
1:1 Relationship
Man
Woman
Peter
Mary
Paul
Linda
John
Nancy
Woody
Mia
Alan
Pia
A man may marry 0 or 1 woman.
A woman may marry 0 or 1 man.
1:M Relationship
Father
Child
Peter
Paul
John
Woody
Alan
A father has 1 or many children.
A child has 1 father.
Mary
Brian
Linda
Aron
Nancy
Ronald
Mia
Pia
Other Examples
• 1:1
– State, State Governor
– Order, Invoice
• 1:M
– Department, Employee
– Customer, Order
• M:M
– Bank customer, Bank account
– Student, Student organization
ERD Notations
1
Student
M
M
Advise
Has
1
Account
M
Enroll
1
Faculty
1
Teach
M
Course
Alternative Notations
Student
Has
Account
Enroll
Advise
Faculty
Teach
Course
Attributes
• Properties of an entity or a relationship.
• Simple attributes vs Composite
attributes
– Address:Street address, City, State,
ZipCode
• Single-valued attributes vs multivalued
attributes
• Derived attributes
• Primary key
FullName
SID
Fname
Lname
DateOfBirth
Age
Student
Major
Domains of Attributes
• The set of allowable values for one or
more attributes.
• Input validation
• Examples:
– Sex: F, M
– EmpHourlyWage: Between 6 and 300
– EmpName: 50 charcters
Introduction to Relational Database
• Data is logically structured within relations.
• Each relation is a table (file) with named
columns (attributes, fields) and rows
(records).
Properties of a Relation
• Simple attribute
– No composite, no multivalued attribute
• Each relation must have a primary key:
– Simple or composite key
– May have other keys (candidate keys)
– Key cannot be null
– Cannot be duplicated
Relational Database Design
• Create a table for each entity that
includes all simple attributes
• Relationship:
– 1:1, 1:M
• Relationship table
• Foreign key
– M:M: relationship table
Database Design Example
• University:
– 1:1 Relationship:
• Student and Account
– What is Peter’s account balance
– Find students that owe more than 2000 dollars.
– 1:M Relationship
• Faculty Advisor and student
– Who is Peter’s advisor?
– How many students advised by Chao?
– M:M Relationship
• Student and course
– How many units Peter takes?
– Find students that are taking 363.
Database Design Example
• Bank:
– Customer, Account, bank employee
Database Design Example
• Order Processing:
– Customer, Order, Product
Aname
Phone
Attorney
AID
CID
Phone
M
M
Cname
Case
M
Hours
1
M
1
Judge
JID
Jname
Court
CtID
Room
Advanced Topics
• Composite key
• Multivalued attribute
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–
–
–
Student’s Major attribute
Faculty’s DegreeEarned attribute
Vehicle’s Color attribute
Others: PhoneNumber, EmailAddress
• Create a table for each multi-valued attribute
• Key + attribute
• Attributes on a relation
Online Shopping Cart
CID
Cname
Customer
Addr
1
Has
CartID
Date
Phone
M
ShoppingCart
M
Has
M
Product
Price
PID
Pname
Order Form
Online Shopping Cart
CID
Addr
Cname
Customer
CartID
Date
Phone
1
Has
M
ShoppingCart
M
Qty
Has
M
Product
Price
PID
Pname
Attributes on Relationship
• Examples:
– Student/Course: Grade
– Order/Product: Quantity