chapter 4 entity relationship (er) modelingcis.csuohio.edu/~sschung/ist331/coronel_ppt_ch04.pdf ·...
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Database SystemsDesign, Implementation, and Management
Coronel | Morris
11e
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Chapter 4
Entity Relationship (ER)
Modeling
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Learning Objectives
� In this chapter, students will learn:
� The main characteristics of entity relationship
components
� How relationships between entities are defined, refined,
and incorporated into the database design process
� How ERD components affect database design and
implementation
� That real-world database design often requires the
reconciliation of conflicting goals
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Entity Relationship Model (ERM)
� Basis of an entity relationship diagram (ERD)
� ERD depicts the:
� Conceptual database as viewed by end user
� Database’s main components
� Entities
� Attributes
� Relationships
� Entity - Refers to the entity set and not to a single
entity occurrence
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Attributes
� Characteristics of entities
� Required attribute: Must have a value, cannot be
left empty
� Optional attribute: Does not require a value, can
be left empty
� Domain - Set of possible values for a given attribute
� Identifiers: One or more attributes that uniquely
identify each entity instance
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Figure 4.1 - The Attributes of the Student
Entity: Chen and Crow’s Foot
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Attributes
� Composite identifier: Primary key composed of more than one attribute
� Composite attribute: Attribute that can be subdivided to yield additional attributes
� Simple attribute: Attribute that cannot be subdivided
� Single-valued attribute: Attribute that has only a single value
� Multivalued attributes: Attributes that have many values
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Figure 4.3 - A Multivalued Attribute in an
Entity
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Attributes
� Multivalued attributes: Attributes that have many
values and require creating:
� Several new attributes, one for each component of the
original multivalued attribute
� A new entity composed of the original multivalued
attribute’s components
� Derived attribute: Attribute whose value is
calculated from other attributes
� Derived using an algorithm
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Figure 4.6 - Depiction of a Derived
Attribute
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Table 4.2 - Advantages and Disadvantages of
Storing Derived Attributes
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Relationships
� Association between entities that always operate in
both directions
� Participants: Entities that participate in a
relationship
� Connectivity: Describes the relationship
classification
� Cardinality: Expresses the minimum and maximum
number of entity occurrences associated with one
occurrence of related entity
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Figure 4.7 - Connectivity and Cardinality in
an ERD
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Existence Dependence
Existence dependence
• Entity exists in the
database only when it is
associated with another
related entity occurrence
Existence independence
• Entity exists apart from
all of its related entities
• Referred to as a strong
entity or regular entity
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Relationship Strength
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Weak (non-identifying) relationship
• Primary key of the related entity does not contain a primary key component of the parent entity
Strong (identifying) relationships
• Primary key of the related entity contains a primary key component of the parent entity
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Figure 4.8 - A Weak (Non-Identifying)
Relationship between COURSE and
CLASS
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A1
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Slide 15
A1 Have left a placeholder as I was working on a secured PDF and the resolution of the screen shot was not good. Author, 1/20/2014
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Figure 4.9 - A Strong (Identifying)
Relationship between COURSE and
CLASS
16
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A2
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Slide 16
A2 Refer comment 1. Author, 1/20/2014
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Weak Entity
� Conditions
� Existence-dependent
� Has a primary key that is partially or totally derived
from parent entity in the relationship
� Database designer determines whether an entity is
weak based on business rules
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Figure 4.10 - A Weak Entity in an ERD
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Figure 4.11 - A Weak Entity in a Strong
Relationship
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Relationship Participation
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Optional participation
• One entity occurrence does not require a corresponding entity occurrence in a particular relationship
Mandatory participation
• One entity occurrence requires a corresponding entity occurrence in a particular relationship
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Table 4.3 - Crow’s Foot Symbols
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Figure 4.13 - CLASS is Optional to
COURSE
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Figure 4.14 - COURSE and CLASS in a
Mandatory Relationship
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Relationship Degree
� Indicates the number of entities or participants
associated with a relationship
� Unary relationship: Association is maintained
within a single entity
� Recursive relationship: Relationship exists between
occurrences of the same entity set
� Binary relationship: Two entities are associated
� Ternary relationship: Three entities are associated
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Figure 4.15 - Three Types of Relationship
Degree
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A3
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Slide 25
A3 Refer comment 1Author, 1/20/2014
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Figure 4.17 - An ER Representation of
Recursive Relationships
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Associative Entities
� Also known as composite or bridge entities
� Used to represent an M:N relationship between two
or more entities
� Is in a 1:M relationship with the parent entities
� Composed of the primary key attributes of each parent
entity
� May also contain additional attributes that play no
role in connective process
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Figure 4.23 - Converting the M:N Relationship
into Two 1:M Relationships
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Figure 4.25 - A Composite Entity in an
ERD
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Developing an ER Diagram
� Create a detailed narrative of the organization’s
description of operations
� Identify business rules based on the descriptions
� Identify main entities and relationships from the
business rules
� Develop the initial ERD
� Identify the attributes and primary keys that
adequately describe entities
� Revise and review ERD
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Figure 4.26 - The First Tiny College ERD
Segment
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Figure 4.27 - The Second Tiny College
ERD Segment
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Figure 4.28 - The Third Tiny College ERD
Segment
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Figure 4.29 - The Fourth Tiny College ERD
Segment
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Figure 4.30 - The Fifth Tiny College ERD
Segment
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Figure 4.31 - The Sixth Tiny College ERD
Segment
36
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Figure 4.32 - The Seventh Tiny College
ERD Segment
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Figure 4.33 - The Eighth Tiny College ERD
Segment
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Figure 4.34 - The Ninth Tiny College ERD
Segment
39
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Table 4.4 - Components of the ERM
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Database Design Challenges:
Conflicting Goals
41
Database design must conform to design standards
Need for high processing speed may limit the number and complexity of logically desirable relationships
Need for maximum information generation may lead to loss of clean design structures and high transaction speed
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Figure 4.38 - Various Implementations of
the 1:1 Recursive Relationship
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