1. Lecture 5 Entity Relationship Modeling
Software Engineering
Lecture 5
Entity Relationship Modeling

2. Entity-Relationship Model
Technique for carrying out the conceptual and logical design of the system
A widely accepted data modelling approach
3 basic notions:
entities
attributes
relationships

3. What is an entity?
An entity is an object that can be identified in the users’ work environment and that users want to track.

4. Entities
An entity is a thing or object in the real world (within the application context)
An entity has a set of properties which uniquely identify it.
An entity is represented as a rectangle in an ER diagram
Project

5. What is an Attribute?
An attribute describes a characteristic of an entity
For example:
An entity: Employee
Has attributes:
Employee_Name
Extension
Date_Of_Hire

6. Attributes Example: Project = (proj_name, location, budget,
start_date, end_date)
Represented as ellipses in an ER diagram
budget
start_date
location
Project
end_date
proj_name

7. What are Relationships?
Relationships are associations between entities which express some real world relationship
Project
Employs
Employee
Project and Employee participate in the employs relationship
The function that an entity plays in a relationship is called that entity’s role

8. Relationship Sets
starts at
There can be more than one relationship between entities.
There can be recursive relationships that can indicate roles for clarity.
A relationship can also have descriptive attributes.
Road
Town
ends at
manager
Employee
works for
worker
date
Client
orders
Book

9. Degree of Relationships
Refers to the number of entities participating in a relationship
Most relationships are binary (degree 2), but can also have ternary (degree 3) relationships.
Name
proj_name
Project
Hires
Contractor
location
Address
budget
end_date
start_date
end_date
Contract
number
start_date
value

10. Mapping Cardinalities
Express the number of entities to which another entity can be associated via a relationship
For a binary relationship, the mapping cardinality can be one of the following:
one to one
one to many
many to one
many to many

11. Cardinality Examples
A person has only one ID book and an ID book belongs to only 1 person
A person can own many cars (possibly 0), but a car only has 1 owner
A person can own many houses and a house can have multiple owners
Person
ID Book
owns
Person
Car
owns
Person
House
owns

12. Design Issues Entity or Attribute?
Example: should location be an entity or an attribute?
can a project be in more than 1 location?
Entity or Relationship?
generally: relationships for actions between entities
whatever “makes sense” in application context

13. ER Diagram Symbols Entity Set One-to-one link Many-to-one link
Attribute
Many-to-many link
Relationship

14. Example ERD Project Employee Alien employs eliminates start_date
budget ID
surname
end_date
first_name
proj_name
Project
employs
Employee
location
position
year_joined
start_date
end_date
eliminates
year_left
species
Alien
area

15. ER Design: Where to Start
Identify entity sets
Describe the entity sets with attributes
Specify relationships and cardinalities

16. Simple Modelling Example
Every department within the company is in only one division.
Each division has more than one department in it.
There is no upper limit on the number of departments that a division can have.

17. 1. Define Entities
Division
Department
Employee

18. 2. Define Relationships
Division
contains
Department
manages
Employee

19. 3. Define Cardinality
Division
contains
Department
manages
Employee

20. Standard Notation A plain rectangle is used to
represent the entity type.
INVOICE
A labeled line to represent the
relationship.
is sent by
1 : 1 A B A B
1 : N

21. Diagram with Standard Notation
Division
contains
Department
manages
Employee

22. Notation Examples ONE course has enrolled ONE or MORE students.
ONE student is enrolled on ONE course.
LOAN
BOOK
refers to
ONE loan refers to ONE book.
ONE book is referred to ONE loan.

23. Many-to-Many Relationship Example
Any one supplier might supply more than one kind of part.
Any one kind of part might be bought from a number of different suppliers.
SUPPLIER
PART
supplies

24. More Examples is associated can place teach Customer Property Customer
Order
teach
Teachers
Students

25. Example - Simple Hospital System
WARD
DOCTOR
has
accommodates
assigned
treats
NURSE
PATIENT
AILMENT
cares
for
suffers
from

26. College System DEPARTMENT COURSE LECTURER STUDENT is_in offers teaches
enrols
STUDENT

27. Your Project Work Develop Requirements Document.
Think about your system’s entities and relationships