Transcript chap4 seq10.ppt

System Analysis and Design
System Design
- Mr. Ahmad Al-Ghoul
learning Objectives
 Describe data
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relationships
Define cardinality, and use
cardinality notation
Draw an entityrelationship diagram
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Entity-Relationship Diagrams
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What Is an ERD?
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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
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Entity-Relationship Diagrams
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Representing entities
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we represent an entity by a named rectangle
use a singular noun, or adjective + noun
refer to one instance in naming
PART-TIME
EMPLOYEE
CUSTOMER
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Entity-Relationship Diagrams
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Entity types and instances
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an entity type is a classification of entity instances
Example Ahmad
Amal
Osama
SUPPLIER
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Entity-Relationship Diagrams
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Relationship
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is an association between two entities
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we may wish to store information about the association
Relationships should have active verb names
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often recognised by a verb or
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"entity + verb + entity"
eg CUSTOMER places ORDER
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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
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Entity-Relationship Diagrams
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A Relationship Type is a relevant business association
between two Entity Types.
A Relationship is an occurrence of a relationship type.
Marketing
Finance
MIS
DEPT
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employs
employs
employs
employs
employs
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Yosef Ali
Osama Mlkawi
Ahmad Al ghoul
EMPLOYEE
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Entity-Relationship Diagrams
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Types of
Relationships
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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)
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Entity-Relationship Diagrams
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Types of
Relationships
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One-to-many
relationship
(1:M): One
instance in an
entity (parent)
refers to one
or more
instances in
the related
entity (child)
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Entity-Relationship Diagrams
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Types of Relationships
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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.
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Entity-Relationship Diagrams
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Types of Relationships
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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-tomany, 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.
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Associative
Entity
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Entity-Relationship Diagrams
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Cardinalities in relationships
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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.
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Entity-Relationship Diagrams
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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.
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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.
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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.
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Entity-Relationship Diagrams
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The entity-relationship diagram may be used to
determine record keys
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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
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Sequence Summary
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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 manyto-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
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Sequence Summary
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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
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Reference
[1] System Analysis and Design, Sixth
Edition
Authors: Gary B. Shelly, Thomas J.
Cashman and Harry J. Rosenblatt
Publisher: SHELLY CASHMAN SEWIES.
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