An entity relationship diagram (ERD) graphically represents the entities in a database and their relationships, showing entity names, relationships, cardinality, and optional attributes. Cardinality describes the numerical relationship between rows in related tables as one-to-one, one-to-many, or many-to-many. Examples show the ERD for employees and companies, and customers, bookings, cars, and depots.

1. ENTITY RELATIONSHIP DIAGRAM
Higher Computing Science

2. ENTITY RELATIONSHIP DIAGRAM
• An ERD is a graphical representation of the entities in a database, and
their relationship to each other
• Indicates:
• Entity name
• Relationship
• Cardinality
• Attribute names (optional)

3. CARDINALITY
Cardinality is a description of the numerical relationship between the rows
of one table and the rows of the other.
There are 3 types of cardinality:
• One-to-one (1:1)
• One-to-many (1:m)
• Many-to-many (m:n)

4. CARDINALITY
Employee Laptop
Company
Project
works on
is allocated
works for
Employee
Employee
One-to-one
relationship
One-to-many
relationship
Many-to-
many
relationship

5. ERD – EMPLOYEE EXAMPLE
Employee Company
works for
Employee
Code
Name
Salary
DOB
Company
Number *
Company
Name
Company
Number
Address
Telephone
Managing
Director

6. ERD – CAR HIRE EXAMPLE
Customer Booking Car Depot
places contain
s
is stored at