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Database Systems:
Design, Implementation, and
Management
Ninth Edition
Chapter 4
Entity Relationship (ER) Modeling
Database Systems, 9th Edition 2
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
Database Systems, 9th Edition 3
The Entity Relationship Model (ERM)
• 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
Database Systems, 9th Edition 4
Entities
• Refers to entity set and not to single entity
occurrence
• Corresponds to table and not to row in
relational environment
• In Chen and Crow’s Foot models, entity is
represented by rectangle with entity’s name
• Entity name, a noun, written in capital letters
Database Systems, 9th Edition 5
Attributes
• Characteristics of entities
• Chen notation: attributes represented by ovals
connected to entity rectangle with a line
– Each oval contains the name of attribute it
represents
• Crow’s Foot notation: attributes written in
attribute box below entity rectangle
Database Systems, 9th Edition 6
Database Systems, 9th Edition 7
Attributes (cont’d.)
• Required attribute: must have a value
• Optional attribute: may be left empty
• Domain: set of possible values for an attribute
– Attributes may share a domain
• Identifiers: one or more attributes that uniquely
identify each entity instance
• Composite identifier: primary key composed
of more than one attribute
Database Systems, 9th Edition 8
Database Systems, 9th Edition 9
Attributes (cont’d.)
• Composite attribute can be subdivided
• Simple attribute cannot be subdivided
• Single-value attribute can have only a single
value
• Multivalued attributes can have many values
Database Systems, 9th Edition 10
Database Systems, 9th Edition 11
Attributes (cont’d.)
• M:N relationships and multivalued attributes should
not be implemented
– Create several new attributes for each of the
original multivalued attributes’ components
– Create new entity composed of original
multivalued attributes’ components
• Derived attribute: value may be calculated
from other attributes
– Need not be physically stored within database
Database Systems, 9th Edition 12
Database Systems, 9th Edition 13
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 only one side of the relationship is known
Database Systems, 9th Edition 14
Connectivity and Cardinality
• Connectivity
– Describes 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
Database Systems, 9th Edition 15
Database Systems, 9th Edition 16
Existence Dependence
• Existence dependence
– Entity exists 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 such an entity is referred to as a
strong or regular entity
Database Systems, 9th Edition 17
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
Database Systems, 9th Edition 18
Database Systems, 9th Edition 19
Database Systems, 9th Edition 20
Weak Entities
• Weak entity meets two conditions
– Existence-dependent
– Primary key partially or totally derived from
parent entity in relationship
• Database designer determines whether an
entity is weak based on business rules
Database Systems, 9th Edition 21
Database Systems, 9th Edition 22
Database Systems, 9th Edition 23
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
Database Systems, 9th Edition 24
Database Systems, 9th Edition 25
Database Systems, 9th Edition 26
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
Database Systems, 9th Edition 27
Database Systems, 9th Edition 28
Database Systems, 9th Edition 29
Recursive Relationships
• Relationship can exist between occurrences of
the same entity set
– Naturally found within unary relationship
Database Systems, 9th Edition 30
Database Systems, 9th Edition 31
Database Systems, 9th Edition 32
Associative (Composite) Entities
• Also known as bridge entities
• Used to implement M:N relationships
• Composed of primary keys of each of the
entities to be connected
• May also contain additional attributes that play
no role in connective process
Database Systems, 9th Edition 33
Database Systems, 9th Edition 34
Database Systems, 9th Edition 35
Developing an ER Diagram
• Database design is an iterative process
– 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
Database Systems, 9th Edition 36
Database Systems, 9th Edition 37
Database Systems, 9th Edition 38
Database Systems, 9th Edition 39
Database Systems, 9th Edition 40
Database Systems, 9th Edition 41
Database Systems, 9th Edition 42
Database Systems, 9th Edition 43
Database Systems, 9th Edition 44
Database Systems, 9th Edition 45
Database Systems, 9th Edition 46
Database Systems, 9th Edition 47
Database Design Challenges:
Conflicting Goals
• Database designers must make design
compromises
– Conflicting goals: design standards, processing
speed, information requirements
• Important to meet logical requirements and
design conventions
• Design is of little value unless it delivers all
specified query and reporting requirements
• Some design and implementation problems do
not yield “clean” solutions
Database Systems, 9th Edition 48
Database Systems, 9th Edition 49
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
Database Systems, 9th Edition 50
Summary (cont’d.)
• Connectivities and cardinalities are based on
business rules
• M:N relationship is valid at conceptual level
– Must be mapped to a set of 1:M relationships
• ERDs may be based on many different ERMs
• UML class diagrams are used to represent the
static data structures in a data model
• Database designers are often forced to make
design compromises

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Fundamentals of Database ppt ch04

  • 1. Database Systems: Design, Implementation, and Management Ninth Edition Chapter 4 Entity Relationship (ER) Modeling
  • 2. Database Systems, 9th Edition 2 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
  • 3. Database Systems, 9th Edition 3 The Entity Relationship Model (ERM) • 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
  • 4. Database Systems, 9th Edition 4 Entities • Refers to entity set and not to single entity occurrence • Corresponds to table and not to row in relational environment • In Chen and Crow’s Foot models, entity is represented by rectangle with entity’s name • Entity name, a noun, written in capital letters
  • 5. Database Systems, 9th Edition 5 Attributes • Characteristics of entities • Chen notation: attributes represented by ovals connected to entity rectangle with a line – Each oval contains the name of attribute it represents • Crow’s Foot notation: attributes written in attribute box below entity rectangle
  • 7. Database Systems, 9th Edition 7 Attributes (cont’d.) • Required attribute: must have a value • Optional attribute: may be left empty • Domain: set of possible values for an attribute – Attributes may share a domain • Identifiers: one or more attributes that uniquely identify each entity instance • Composite identifier: primary key composed of more than one attribute
  • 9. Database Systems, 9th Edition 9 Attributes (cont’d.) • Composite attribute can be subdivided • Simple attribute cannot be subdivided • Single-value attribute can have only a single value • Multivalued attributes can have many values
  • 10. Database Systems, 9th Edition 10
  • 11. Database Systems, 9th Edition 11 Attributes (cont’d.) • M:N relationships and multivalued attributes should not be implemented – Create several new attributes for each of the original multivalued attributes’ components – Create new entity composed of original multivalued attributes’ components • Derived attribute: value may be calculated from other attributes – Need not be physically stored within database
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  • 13. Database Systems, 9th Edition 13 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 only one side of the relationship is known
  • 14. Database Systems, 9th Edition 14 Connectivity and Cardinality • Connectivity – Describes 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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  • 16. Database Systems, 9th Edition 16 Existence Dependence • Existence dependence – Entity exists 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 such an entity is referred to as a strong or regular entity
  • 17. Database Systems, 9th Edition 17 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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  • 20. Database Systems, 9th Edition 20 Weak Entities • Weak entity meets two conditions – Existence-dependent – Primary key partially or totally derived from parent entity in relationship • Database designer determines whether an entity is weak based on business rules
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  • 23. Database Systems, 9th Edition 23 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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  • 26. Database Systems, 9th Edition 26 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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  • 29. Database Systems, 9th Edition 29 Recursive Relationships • Relationship can exist between occurrences of the same entity set – Naturally found within unary relationship
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  • 32. Database Systems, 9th Edition 32 Associative (Composite) Entities • Also known as bridge entities • Used to implement M:N relationships • 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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  • 35. Database Systems, 9th Edition 35 Developing an ER Diagram • Database design is an iterative process – 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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  • 47. Database Systems, 9th Edition 47 Database Design Challenges: Conflicting Goals • Database designers must make design compromises – Conflicting goals: design standards, processing speed, information requirements • Important to meet logical requirements and design conventions • Design is of little value unless it delivers all specified query and reporting requirements • Some design and implementation problems do not yield “clean” solutions
  • 48. Database Systems, 9th Edition 48
  • 49. Database Systems, 9th Edition 49 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
  • 50. Database Systems, 9th Edition 50 Summary (cont’d.) • Connectivities and cardinalities are based on business rules • M:N relationship is valid at conceptual level – Must be mapped to a set of 1:M relationships • ERDs may be based on many different ERMs • UML class diagrams are used to represent the static data structures in a data model • Database designers are often forced to make design compromises