chapter 2: modeling data in the organization © 2013 pearson education, inc. publishing as prentice...
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CHAPTER 2:CHAPTER 2:MODELING DATA IN THE MODELING DATA IN THE ORGANIZATIONORGANIZATION
© 2013 Pearson Education, Inc. Publishing as Prentice Hall
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Modern Database Management11th Edition
Jeffrey A. Hoffer, V. Ramesh, Heikki Topi
Chapter 2 © 2013 Pearson Education, Inc. Publishing as Prentice Hall© 2013 Pearson Education, Inc. Publishing as Prentice Hall
OBJECTIVESOBJECTIVES
Define termsDefine terms Write good names and definitions for Write good names and definitions for
entities, relationships, and attributesentities, relationships, and attributes Model different types of attributes and Model different types of attributes and
entities, entities, Introduce the concept of relationships, Introduce the concept of relationships,
and cardinalitiesand cardinalities Draw E-R diagrams for common business Draw E-R diagrams for common business
situationssituations
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A GOOD DATA NAME IS:A GOOD DATA NAME IS:
Related to bRelated to business, not technical, usiness, not technical, characteristicscharacteristics
Meaningful and self-documentingMeaningful and self-documenting UniqueUnique Written in standard syntaxWritten in standard syntax
Data DefinitionsData Definitions Provide a concise description of Provide a concise description of
essential data meaningessential data meaning
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E-R MODEL E-R MODEL CONSTRUCTSCONSTRUCTS
Entities:Entities: Entity instance–person, place, object, event, concept Entity instance–person, place, object, event, concept
(often corresponds to a row in a table)(often corresponds to a row in a table) Entity Type–collection of entities (often corresponds to Entity Type–collection of entities (often corresponds to
a table)a table) Relationships:Relationships:
Relationship instance–link between entity instances Relationship instance–link between entity instances (corresponds to primary key-foreign key equivalencies in (corresponds to primary key-foreign key equivalencies in related tables)related tables)
Relationship type–category of relationship…link between entity Relationship type–category of relationship…link between entity typestypes
Attributes:Attributes: Properties or characteristics of an entity or relationship Properties or characteristics of an entity or relationship
type (often corresponds to a field in a table)type (often corresponds to a field in a table)
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ENTITY TYPE AND ENTITY ENTITY TYPE AND ENTITY INSTANCESINSTANCES
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AN ENTITY…AN ENTITY…
SHOULD BE:SHOULD BE: An object that will have many An object that will have many
instances in the databaseinstances in the database An object that will be composed of An object that will be composed of
multiple attributesmultiple attributes An object that we are trying to modelAn object that we are trying to model
SHOULD NOT BE:SHOULD NOT BE: A user of the database system A user of the database system An output of the database system An output of the database system
(e.g., a report)(e.g., a report)
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Inappropriate entities
System System useruser
System System outputoutput
Figure 2-4 Example of inappropriate entities
Appropriate entities
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ATTRIBUTESATTRIBUTES
Attribute–property or characteristic of Attribute–property or characteristic of an entity or relationship typean entity or relationship type
Classifications of attributes:Classifications of attributes: Required versus Optional AttributesRequired versus Optional Attributes Simple versus Composite AttributeSimple versus Composite Attribute Single-Valued versus Multivalued Single-Valued versus Multivalued
AttributeAttribute Stored versus Derived AttributesStored versus Derived Attributes Identifier AttributesIdentifier Attributes
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SIMPLE VS. COMPOSITE SIMPLE VS. COMPOSITE ATTRIBUTESATTRIBUTES
Composite attributeComposite attribute – An attribute that has – An attribute that has meaningful component parts (attributes)meaningful component parts (attributes)
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The address is broken into component parts
Figure 2-7 A composite attribute
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Figure 2-8 Entity with multivalued attribute (Skill) and derived attribute (Years Employed)
Multivaluedan employee can have more than one skill
DerivedCalculated from date employed and current date
Multi-valued and Derived Multi-valued and Derived AttributesAttributes
Multivalued – may take on more than one value for a given entity (or relationship) instance
Derived – values can be calculated from related attribute values (not physically stored in the database)
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Chapter 2 © 2013 Pearson Education, Inc. Publishing as Prentice Hall© 2013 Pearson Education, Inc. Publishing as Prentice Hall
IDENTIFIERS (KEYS)IDENTIFIERS (KEYS)
Identifier (Key)–an attribute (or Identifier (Key)–an attribute (or combination of attributes) that combination of attributes) that uniquely identifies individual uniquely identifies individual instances of an entity typeinstances of an entity type
Simple versus Composite IdentifierSimple versus Composite Identifier Candidate Identifier–an attribute Candidate Identifier–an attribute
that could be a key…satisfies the that could be a key…satisfies the requirements for being an identifierrequirements for being an identifier
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CRITERIA FOR IDENTIFIERSCRITERIA FOR IDENTIFIERS
Choose Identifiers thatChoose Identifiers that Will not change in valueWill not change in value Will not be nullWill not be null
Avoid intelligent identifiers (e.g., Avoid intelligent identifiers (e.g., containing locations or people that might containing locations or people that might change)change)
Substitute new, simple keys for long, Substitute new, simple keys for long, composite keyscomposite keys
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BUSINESS RULESBUSINESS RULES Are statements that define or constrain Are statements that define or constrain
some aspect of the businesssome aspect of the business Are derived from policies, procedures, Are derived from policies, procedures,
events, functionsevents, functions Assert business structureAssert business structure Control/influence business behaviorControl/influence business behavior Are expressed in terms familiar to end usersAre expressed in terms familiar to end users Are automated through DBMS softwareAre automated through DBMS software
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If possible
Chapter 2 © 2013 Pearson Education, Inc. Publishing as Prentice Hall© 2013 Pearson Education, Inc. Publishing as Prentice Hall
E-R MODEL E-R MODEL CONSTRUCTSCONSTRUCTS
Entities:Entities: Entity instance–person, place, object, event, concept (often Entity instance–person, place, object, event, concept (often
corresponds to a row in a table)corresponds to a row in a table) Entity Type–collection of entities (often corresponds to a Entity Type–collection of entities (often corresponds to a
table)table) Relationships:Relationships:
Relationship instance–link between entity instances Relationship instance–link between entity instances (corresponds to primary key-foreign key (corresponds to primary key-foreign key equivalencies in related tables)equivalencies in related tables)
Relationship type–category of relationship…link Relationship type–category of relationship…link between entity typesbetween entity types
Attributes:Attributes: Properties or characteristics of an entity or relationship type Properties or characteristics of an entity or relationship type
(often corresponds to a field in a table)(often corresponds to a field in a table)
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MODELING RELATIONSHIPSMODELING RELATIONSHIPS
Relationship Types vs. Relationship Relationship Types vs. Relationship InstancesInstances The relationship type is modeled as lines The relationship type is modeled as lines
between entity types…the instance is between entity types…the instance is between specific entity instancesbetween specific entity instances
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Figure 2-10 Relationship types and instances
a) Relationship type (Completes)
b) Relationship instances
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Sample E-R Diagram (Figure 2-1)
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Degree of relationships – from Figure 2-2
Entities of two different types related to each other
Entities of three different types related to each other
One entity related to another of the same entity type
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Chapter 2 © 2013 Pearson Education, Inc. Publishing as Prentice Hall© 2013 Pearson Education, Inc. Publishing as Prentice Hall
CARDINALITY OF RELATIONSHIPSCARDINALITY OF RELATIONSHIPS
One-to-OneOne-to-One Each entity in the relationship will have exactly Each entity in the relationship will have exactly
one related entityone related entity One-to-ManyOne-to-Many
An entity on one side of the relationship can An entity on one side of the relationship can have many related entities, but an entity on have many related entities, but an entity on the other side will have a maximum of one the other side will have a maximum of one related entityrelated entity
Many-to-ManyMany-to-Many Entities on both sides of the relationship can Entities on both sides of the relationship can
have many related entities on the other sidehave many related entities on the other side
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CARDINALITY CONSTRAINTSCARDINALITY CONSTRAINTS Cardinality Constraints—the number of Cardinality Constraints—the number of
instances of one entity that can or must instances of one entity that can or must be associated with each instance of be associated with each instance of another entityanother entity
Minimum CardinalityMinimum Cardinality If zero, then optionalIf zero, then optional If one or more, then mandatoryIf one or more, then mandatory
Maximum CardinalityMaximum Cardinality The maximum numberThe maximum number
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Figure 2-17 Examples of cardinality constraints
a) Mandatory cardinalities
A patient must have recorded at least one history, and can have many
A patient history is recorded for one and only one patient
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Figure 2-17 Examples of cardinality constraints (cont.)
b) One optional, one mandatory
An employee can be assigned to any number of projects, or may not be assigned to any at all
A project must be assigned to at least one employee, and may be assigned to many
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Figure 2-12 Examples of relationships of different degrees (cont.)
b) Binary relationships
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