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Chapter 4
Entity Relationship (ER) Modeling
Database Systems:
Design, Implementation, and Management,
Seventh Edition, Rob and Coronel
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In this chapter, you will learn:
• The main characteristics of entity relationship
components
• How relationships between entities are defined and
refined and how those relationships are 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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The Entity Relationship (ER) Model
• ER model forms the basis of an ER diagram
• ERD represents conceptual database as
viewed by end user
• ERDs depict database’s main components:
– Entities
– Attributes
– Relationships
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Entities
• Refers to entity set and not to single entity
occurrence
• Corresponds to table and not to row in relational
environment
• In both Chen and Crow’s Foot models, entity is
represented by rectangle containing entity’s
name
• Entity name, a noun, is usually written in capital
letters
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Attributes
• Characteristics of entities
• In Chen model, attributes are represented by
ovals and are connected to entity rectangle with
a line
• Each oval contains the name of attribute it
represents
• In Crow’s Foot model, attributes are written in
attribute box below entity rectangle
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Attributes (continued)
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Domains
• Attributes have domain
– Domain is attribute’s set of possible values
• Attributes may share a domain
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Identifiers (Primary Keys)
• Underlined in the ERD
• Key attributes are also underlined in
frequently used table structure shorthand
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Composite Primary Keys
• Primary keys ideally composed of only single
attribute
• Possible to use a composite key
– Primary key composed of more than one
attribute
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Composite Primary Keys (continued)
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Composite and Simple Attributes
• Composite attribute can be subdivided
• Simple attribute cannot be subdivided
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Single-Valued Attributes
• Single-value attribute can have only a single
value
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Multivalued Attributes
• Multivalued attributes can have many values
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Multivalued Attributes (continued)
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Resolving Multivalued Attribute Problems
• Although conceptual model can handle M:N
relationships and multivalued attributes, you
should not implement them in relational DBMS
– Within original entity, create several new
attributes, one for each of the original multivalued
attribute’s components
• Can lead to major structural problems in table
– Create new entity composed of original
multivalued attribute’s components
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Resolving Multivalued Attribute Problems
(continued)
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Resolving Multivalued Attribute Problems
(continued)
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Resolving Multivalued Attribute Problems
(continued)
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Derived Attributes
• Attribute whose value may be calculated
(derived) from other attributes
• Need not be physically stored within
database
• Can be derived by using an algorithm
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Derived Attributes (continued)
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Derived Attributes (continued)
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Relationships
• Association between entities
• Participants are entities that participate in a
relationship
• Relationships between entities always operate in
both directions
• Relationship can be classified as 1:M
• Relationship classification is difficult to establish if
know only one side of the relationship
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Connectivity and Cardinality
• Connectivity
– Used to describe the relationship classification
• Cardinality
– Expresses minimum and maximum number of
entity occurrences associated with one
occurrence of related entity
• Established by very concise statements
known as business rules
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Connectivity and Cardinality (continued)
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Existence Dependence
• Existence dependence
– Exist in database only when it is associated
with another related entity occurrence
• Existence independence
– Entity can exist apart from one or more related
entities
– Sometimes refers to such an entity as strong
or regular entity
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Relationship Strength
• Weak (non-identifying) relationships
– Exists if PK of related entity does not contain
PK component of parent entity
• Strong (Identifying) Relationships
– Exists when PK of related entity contains PK
component of parent entity
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Weak (Non-Identifying) Relationships
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Weak (Non-Identifying)
Relationships (continued)
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Strong (Identifying) Relationships
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Weak Entities
• Weak entity meets two conditions
– Existence-dependent
• Cannot exist without entity with which it has a
relationship
– Has primary key that is partially or totally derived
from parent entity in relationship
• Database designer usually determines whether
an entity can be described as weak based on
business rules
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Weak Entities (continued)
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Weak Entities (continued)
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Relationship Participation
• Optional participation
– One entity occurrence does not require
corresponding entity occurrence in particular
relationship
• Mandatory participation
– One entity occurrence requires corresponding
entity occurrence in particular relationship
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Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Participation (continued)
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Relationship Degree
• Indicates number of entities or participants
associated with a relationship
• Unary relationship
– Association is maintained within single entity
• Binary relationship
– Two entities are associated
• Ternary relationship
– Three entities are associated
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Relationship Degree (continued)
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Relationship Degree (continued)
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Recursive Relationships
• Relationship can exist between occurrences
of the same entity set
• Naturally found within unary relationship
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Recursive Relationships
(continued)
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Recursive Relationships
(continued)
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Recursive Relationships
(continued)
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Recursive Relationships
(continued)
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Recursive Relationships
(continued)
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Recursive Relationships
(continued)
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Composite Entities
• Also known as bridge entities
• Composed of primary keys of each of the
entities to be connected
• May also contain additional attributes that
play no role in connective process
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Composite Entities (continued)
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Composite Entities (continued)
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Composite Entities (continued)
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Developing an ER Diagram
• Database design is iterative rather than linear
or sequential process
• Iterative process
– Based on repetition of processes and
procedures
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Developing an ER Diagram (continued)
• Building an ERD usually involves the following
activities:
– Create detailed narrative of organization’s description
of operations
– Identify business rules based on description of
operations
– Identify main entities and relationships from business
rules
– Develop initial ERD
– Identify attributes and primary keys that adequately
describe entities
– Revise and review ERD
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Developing an ER Diagram (continued)
• Tiny College
– Tiny College is divided into several schools
• Each school is composed of several departments
– Each department may offer courses
– Each department may have many professors assigned to
it
– Each professor may teach up to four classes; each class
is section of course
– Student may enroll in several classes, but (s)he takes
each class only once during any given enrollment period
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Developing an ER Diagram (continued)
• Tiny College (continued)
– Each department has several students
• Each student has only a single major and is associated with
a single department
– Each student has an advisor in his or her department
• Each advisor counsels several students
– The relationship between class is taught in a room and
the room in the building
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Developing an ER Diagram (continued)
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Database Design Challenges:
Conflicting Goals
• Database design must conform to design
standards
• High processing speeds are often a top
priority in database design
• Quest for timely information might be focus of
database design
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Database Design Challenges:
Conflicting Goals (continued)
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Summary
• Entity relationship (ER) model
– Uses ERD to represent conceptual database
as viewed by end user
– ERM’s main components:
• Entities
• Relationships
• Attributes
– Includes connectivity and cardinality notations
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Summary (continued)
• Connectivities and cardinalities are based on
business rules
• In ERM, M:N relationship is valid at
conceptual level
• ERDs may be based on many different ERMs
• Database designers are often forced to make
design compromises
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