1. ERD diagram Data Design - Mr. Ahmad Al-Ghoul
In this sequence we will discuss the ERD

2. learning Objectives Describe data relationships
Define cardinality, and use cardinality notation
Draw an entity-relationship diagram

6. Entity-Relationship Diagrams
What Is an ERD?
A picture showing the information created, stored, and used by a business system.
ERD is network model that describes stored data of a system at a high level of abstraction.
Entities generally represent similar kinds of information
Lines drawn between entities show relationships among the data
High level business rules are also shown

7. Entity-Relationship Diagrams
Representing entities
we represent an entity by a named rectangle
use a singular noun, or adjective + noun
refer to one instance in naming
CUSTOMER
PART-TIME
EMPLOYEE

8. Entity-Relationship Diagrams
Entity types and instances
an entity type is a classification of entity instances
Example Ahmad
Amal
Osama
SUPPLIER

9. Entity-Relationship Diagrams
is an association between two entities
we may wish to store information about the association
Relationships should have active verb names
often recognised by a verb or
"entity + verb + entity"
eg CUSTOMER places ORDER
relationships capture the "business rules" of the system
The first entity in the relationship is the parent entity; the second entity in the relationship is the child entity
Relationships go in both directions

10. Entity-Relationship Diagrams
A Relationship Type is a relevant business association between two Entity Types.
A Relationship is an occurrence of a relationship type.
Marketing
employs
Yosef Ali
Finance
employs
Osama Mlkawi
MIS
employs
Ahmad Al ghoul
DEPT
EMPLOYEE
employs
employs

11. Entity-Relationship Diagrams
Types of Relationships
Three types of relationships can exist between entities
One-to-one relationship (1:1): One instance in an entity (parent) refers to one and only one instance in the related entity (child)

12. Entity-Relationship Diagrams
Types of Relationships
One-to-many relationship (1:M): One instance in an entity (parent) refers to one or more instances in the related entity (child)

13. Entity-Relationship Diagrams
Types of Relationships
Many-to-many relationship (M:N): exists when one instance of the first entity (parent) can relate to many instances of the second entity (child), and one instance of the second entity can relate to many instances of the first entity.

14. Entity-Relationship Diagrams
Types of Relationships
Many-to-many relationship (M:N):
this relationship can not represent physically so we solve this problem by dividing it to two (1:M) relationships.
The relationship between STUDENT and CLASS is many-to-many, one student can take many classes, and one class can have many students.
To solve the problem of this relationship note that the event or transaction that links the two entities is actually a third entity, called Associative entity that has its own characteristics.
In the next section (normalization) you will see how we will solve this problem.
Associative Entity
The ENROLLS IN symbol represent a REGISTRATION entity that records each instance of a specific student enrolling in a specific course.
Similarly, the RESERVES SEAT ON symbol represents a RESERVATION entity that records each instance of a specific passenger reserving a seat on a specific flight

15. Entity-Relationship Diagrams
Cardinalities in relationships
The cardinality of a relationship is the number of instances of one entity type that may be associated with each instance of the other entity type.
An analyst can model this interaction by adding cardinality notation, which uses special symbols to represent the relationship.

16. Entity-Relationship Diagrams
Cardinality
Cardinality notation: Notation that shows relationships between entities.
Crow’s foot notation: A type of cardinality notation. It is called crow's foot notation because of the shapes, which include circles, bars, and symbols, that indicate various possibilities. A single bar indicates one, a double bar indicates one and only one, a circle indicates zero, and a crow's foot indicates many.

17. Entity-Relationship Diagrams
Crow's foot notation is a common method of indicating cardinality. The four examples show how you can use various symbols to describe the relationships between entities.

18. Entity-Relationship Diagrams
In the first example of cardinality notation, one and only one CUSTOMER can place anywhere from zero to many of the ORDER entity. In the second example, one and only one ORDER can include one ITEM ORDERED or many. In the third example, one and only one EMPLOYEE can have one SPOUSE or none. In the fourth example, one EMPLOYEE, or many employees, or none, can be assigned to one PROJECT, or many projects, or none.

19. Entity-Relationship Diagrams
The entity-relationship diagram may be used to determine record keys
When the relationship is one-to-many, the primary key of the file at the one end of the relationship should be contained as a foreign key on the file at the many end of the relationship
A many-to-many relationship should be divided into two one-to-many relationships with an associative entity in the middle

20. Sequence Summary
An entity-relationship diagram (ERD) is a graphic representation of all system entities and the relationships among them
The ERD is based on entities and data stores in DFDs prepared during the systems analysis phase
The three basic relationships represented in an ERD are one-to-one (1:1), one-to-many (1:M), and many-to-many (M:N)
In a M:N relationship, the two entities are linked by an associative entity
The relationship between two entities also is referred to as cardinality
A common form if cardinality notation is called crow’s foot notation, which uses various symbols to describe the characteristics of the relationship

21. Sequence Summary In this Sequence we have
Defined entity relationship diagram (ERD)
Defined and distinguished between entity type and entity instance
Defined and distinguished between relationship and relationship type
Explained types of relationships
Defined cardinality, and used cardinality notation

22. Reference [1] System Analysis and Design, Sixth Edition
Authors: Gary B. Shelly, Thomas J. Cashman and Harry J. Rosenblatt
Publisher: SHELLY CASHMAN SEWIES.