This document discusses data modeling using the entity-relationship model. It covers relationship types (unary, binary, ternary), examples of relationships, and constraints on relationships including cardinality ratio, participation, and multiplicity. Cardinality ratios include one-to-one, one-to-many, many-to-one, and many-to-many. Participation can be mandatory or optional. Multiplicity specifies the minimum and maximum number of entities that can be related.

1. Fundamentals of Database Systems
1
Lecture 8: Data Modeling Using the Entity-Relationship
Model (Part 3).

2. Quick Review of
Lecture 7
2

3. Relationship
• association between two (or more) entities
• relationships represented by diamond
Examples
• Students Register for Subjects
• School Has Staff
• Driver Commits Traffic Offence
• Customer Orders Product
• Supplier Supplies Parts To Projects
3
ENTITY Relationship ENTITY
attribute
attribute
primary key

4. Attributes of Relationship Types
Relationship types can also have attributes.
• E.g. customer orders a product
• quantity is attribute of orders relationship
• E.g. Employee WORKS-ON Project
• To record the number of hours per week that an employee works
on a particular project. The attribute Hours can be included to the
WORKS-ON relationship type.
4
customer product
orders
quantity
Employee Project
works-on
Hours

5. UNARY : One entity set, recursive.
BINARY : Two entity sets linked (mostly used).
TERNARY :Three entity sets linked.
SUBJECT prerequisite
N
M
STUDENT SUBJECT
enrols in
M N
RENTAL SHOP VIDEO
owns
M N
BORROWER
1
Examples of Relationship
5

6. More Examples
6

7. End of Review
7

8. Constraints on Relationships
• There are three types of relationship constraints:
• Cardinality ratio.
• Participation.
• Multiplicity.
8
Relationship Relationship
1 M
(1,*) (1,*)

9. Cardinality Ratio
Cardinality ratio Express the number of relationship an entity can
participate in.
Relationships can be classified as :
one-to-one (1:1)
one-to-many (1:M)
Many-to-one (M:1)
many-to-many (M:N)
9

10. One-to-one Relationship (1:1)
Professor Manage Department
P1 r1 D001
P2
P3 r2 D002
Each professor manages at most one department; and each
department is managed by only one professor 10
Professor Department
Manage
1
1

11. One-to-Many Relationship (1:M)
Professor teach Section
P1 r1 3C01
r2 3C03
P2 r3 3C02
Each professor teaches many sections; but each section is
taught by only one professor (or at most one professor) 11
Professor Section
teach
M
1

12. Many-to-One Relationship (M:1)
Professor work-in Department
P1 r1 Com. Science
P2 r2 Network
P3 r3 Info. Sys
Each professor works in only one Department; but each
Department has many professors (many professors work in one
department)
12
Professor Department
Work-in
1
M

13. Many-to-Many Relationship
Student Enroll-in Course
S1 r1 is300
S2 r2 is400
S3 r3 is500
S4 r4 is600
Each student can enroll in many courses; and each course can be
enrolled by many students. 13
Student course
Enroll-in
M
M

14. Multiplicity
Multiplicity is the number (range) of possible entities that
may relate to a single association through a particular
relationship
• Takes the form (min#, max#)
14

15. Professor Manage Department
P1 r1 D001
P2
P3 r2 D002
• Each Professor may manage 0 to 1 department,
• Each Department should be managed by only one Professor .
Multiplicity
15
Professor Department
manage
(0,1) (1,1)
1 1

16. Multiplicity
Professor teach Section
P1 r1 3C01
P2 r2 3C03
P3 r3 3C02
• Each Professor may teach 0 to 5 Sections,
• Each Section should be taught by only one Professor .
• * = many .. Use it if the number is not specified 16
Professor Section
teach
M
1
(0,5) (1,1)
(0,*)

17. Multiplicity
Student Enroll-in Course
S1 r1 is300
S2 r2 is400
S3 r3 is500
S4 r4 is600
• Each student should enroll in 1 to 7 Courses,
• Each Course may be enrolled by 0 to 100 students.
• * = many .. Use it if the number is not specified 17
Student Course
Enroll-in
M
M
(1,7) (0,100)
(1,*) (0,*)

18. Participation
• Participation constraints determine whether all or only some
entities participate in a relationship.
• Two type of participation:
• Mandatory (Total)
• Optional (Partial)
18

19. Participation
• Optional (partial) (0:*)
• eg, an employee may not have a spouse
• not every staff manages a department
• Partial participation is represented by single line
• Mandatory (total) (1:*)
• eg, every spouse must be that of an employee
• Every department is managed by some staff.
• Mandatory participation is represented by double line
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R E
R E

20. Examples
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Employee Spouse
1 1
has
(0,M) (1,1)
(1:M) (0:N)
Order
Customer place
Subject
Student enrol
1 N
M N

21. 21