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Database Systems: Design, Implementation, and Management Ninth Edition






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Database Systems: Design, Implementation, and Management Ninth Edition. Chapter 4 Entity Relationship (ER) Modeling. Objectives. In this chapter, students will learn: The main characteristics of entity relationship components
Database Systems: Design, Implementation, and Management Ninth Edition

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Database Systems: Design, Implementation, and ManagementNinth Edition

Chapter 4

Entity Relationship (ER) Modeling

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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

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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

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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 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

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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

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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

  • Compositeidentifier: primary key composed of more than one attribute

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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

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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

  • Derivedattribute: value may be calculated from other attributes

    • Need not be physically stored within database

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Database Systems, 9th Edition

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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 only one side of the relationship is known

Database Systems, 9th Edition

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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

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Database Systems, 9th Edition

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Existence Dependence

  • Existencedependence

    • Entity exists in database only when it is associated with another related entity occurrence

  • Existenceindependence

    • 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

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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 Entities

  • Weakentity 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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Database Systems, 9th Edition

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Relationship Participation

  • Optionalparticipation

    • One entity occurrence does not require corresponding entity occurrence in particular relationship

  • Mandatoryparticipation

    • One entity occurrence requires corresponding entity occurrence in particular relationship

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Relationship Degree

  • Indicates number of entities or participants associated with a relationship

  • Unaryrelationship

    • Association is maintained within single entity

  • Binaryrelationship

    • Two entities are associated

  • Ternaryrelationship

    • Three entities are associated

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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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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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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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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

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Database Systems, 9th Edition

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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

Database Systems, 9th Edition

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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

Database Systems, 9th Edition


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