ERD Hoffer

8/12/2019 ERD Hoffer

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Modern Systems Analysisand DesignFourth Edition

Jeffrey A. HofferJoey F. George

Joseph S. Valacich

2005 by Prentice Hall 2005 by Prentice Hall

ap erStructuring System Data

Requirements


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Learning ObjectivesDefine key data modeling terms.Draw entity-relationship (E-R) and class diagrams torepresent common business situations.Explain the role of conceptual data modeling in IS

2005 by Prentice Hall 2005 by Prentice Hall9-2

ana ys s an es gn.Distinguish between unary, binary, and ternaryrelationships.Define four types of business rules.

Compare the capabilities of class diagrams vs. E-Rdiagrams.Relate data modeling to process and logic modeling.


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Conceptual Data Modeling

A detailed model that captures theoverall structure of data in anorganization


Independent of any databasemanagement system (DBMS) or otherimplementation considerations


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Process of Conceptual DataModeling

Develop a data model for the current systemDevelop a new conceptual data model thatincludes all requirements of the new system


In the design stage, the conceptual datamodel is translated into a physical designProject repository links all design and data

modeling steps performed during SDLC


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Deliverables and OutcomePrimary deliverable is an entity-relationship(E-R) diagram or class diagramAs many as 4 E-R or class diagrams areproduced and analyzed


-projects applicationE-R diagram for the application being replacedE-R diagram for the whole database from whichthe new applications data are extractedE-R diagram for the whole database from whichdata for the application system being replaced isdrawn


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Deliverables and Outcome (cont.)

Second deliverable is a set of entries aboutdata objects to be stored in repository orproject dictionary.


epos ory n s a a, process, an og c mo e sof an information system.Data elements included in the DFD must appear inthe data model and vice versa.

Each data store in a process model must relate tobusiness objects represented in the data model.


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Gathering Information forConceptual Data Modeling

Two perspectivesTop-down

Data model is derived from an intimate


understanding of the business.Bottom-up

Data model is derived by reviewing

specifications and business documents.


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Requirements DeterminationQuestions for Data ModelingWhat are subjects/objects of the business?

Data entities and descriptionsWhat unique characteristics distinguish betweensubjects/objects of the same type?


Primary keysWhat characteristics describe each subject/object?

Attributes and secondary keys

How do you use the data?Security controls and user access privileges


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Requirements DeterminationQuestions for Data Modeling (cont.)

Over what period of time are you interested in the

data?Cardinality and time dimensionsAre all instances of each object the same?


, ,What events occur that imply associations betweenobjects?

Relationships and cardinalities

Are there special circumstances that affect the wayevents are handled?Integrity rules, cardinalities, time dimensions


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Introduction to Entity-Relationship (E-R) Modeling

Entity-Relationship (E-R) DiagramA detailed, logical representation of theentities, associations and data elements for


an organization or businessNotation uses three main constructs

Data entitiesRelationshipsAttributes


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Association

between theinstances of one ormore entity types


Person, place, object,event or concept aboutwhich data is to bemaintained

Entity type : collection ofentities with commoncharacteristicsEntity instance : single

entity

named property orcharacteristic of anentity


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Identifier Attributes

Candidate keyAttribute (or combination of attributes) thatuniquely identifies each instance of an


entity typeIdentifier

A candidate key that has been selected asthe unique identifying characteristic for anentity type


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Identifier Attributes(cont.)

Selection rules for an identifier1. Choose a candidate key that will not

change its value.


2. Choose a candidate key that will never benull.

3. Avoid using intelligent keys.

4. Consider substituting single valuesurrogate keys for large composite keys.


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Multivalued Attributes

An attribute that may take on more thanone value for each entity instanceRe resented on E-R Dia ram in two


ways:

double-lined ellipseweak entity


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Entity and Attribute Example

Simple attributes


Identifier attributeeach employee hasa unique ID.

Multivalued attributean employee may havemore than one skill.


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Degree of RelationshipDegree: number of entity types that participate in a

relationshipThree casesUnary: between two instances of one entity type


Ternary: among the instances of three entity types


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CardinalityThe number of instances of entity B that can or must

be associated with each instance of entity AMinimum CardinalityThe minimum number of instances of entity B that may beassociated with each instance of entity A


Maximum CardinalityThe maximum number of instances of entity B that may beassociated with each instance of entity A

Mandatory vs. Optional Cardinalities

Specifies whether an instance must exist or can be absent inthe relationship


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Cardinality Symbols



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Unary Relationship Example



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Binary Relationship Examples



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Associative EntitiesAn entity type that associates the instances ofone or more entity types and containsattributes that are peculiar to the relationshipbetween those entit instances


An associative entity is:An entityA relationship

This is the preferred way of illustrating arelationship with attributes


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A relationship with an attribute


as an associative entity


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Ternary relationship


as an associative entity


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A relationshipthat itself isrelated toother entities


relationshipmust berepresentedas anassociativeentity.


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Supertypes and Subtypes

Subtype: a subrouping of the entities inan entity type that shares commonattributes or relationships distinct from


other subtypesSupertype: a generic entity type thathas a relationship with one or moresubtype


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Rules for Supertype/SubtypesRelationships

Total specialization: an entity instance of thesupertype must be an instance of one of thesubtypesPartial specialization: an entity instance of the


supertype may or may not e an nstance oone of the subtypesDisjoint: an entity instance of the supertypecan be an instance of only one subtype

Overlap: an entity instance of the supertypemay be an instance of multiple subtypes


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Business RulesSpecifications that preserve the integrity ofthe logical data modelFour types


Entity integrity: unique, non-null identifiersReferential integrity constraints: rules governingrelationshipsDomains: valid values for attributesTriggering operations: other business rulesregarding attribute values


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Domains

The set of all data types and ranges ofvalues that an attribute can assumeSeveral advantages


1. Verify that the values for an attribute arevalid2. Ensure that various data manipulation

operations are logical3. Help conserve effort in describing

attribute characteristics


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Triggering OperationsAn assertion or rule that governs the validity of data

manipulation operations such as insert, update anddeleteComponents:


the triggerEvent: data manipulation operation that initiates theoperationEntity Name: name of entity being accessed or modifiedCondition: condition that causes the operation to betriggeredAction: action taken when the operation is triggered


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Packaged Data ModelsGeneric data models that can beapplied and modified for an organizationTwo categories


UniversalIndustry-specificBenefits

Reduced implementation time and costHigh-quality modeling


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Packaged data modelsprovide generic models


a can e cus om zefor a particularorganizations businessrules


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Object Modeling Using ClassDiagrams

Object-oriented approachBased on Unified Modeling Language(UML)


FeaturesObjects and classesEncapsulation of attributes and operations

PolymorphismInheritance


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ObjectsObject: an entity with a well-defined rolein an applicationEach object has:


,values, and relationships of an objectBehavior: represents how an object actsand reacts

Identity: uniqueness, no two objects arethe same


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Classes

Class: a logical grouping of objects withsimilar attributes and behaviorsO eration: a function or service


provided by all instances of a classEncapsulation: the technique of hidinginternal implementation details of anobject from external view


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Class DiagramA diagram showing the static structure

of an object-oriented model


UML classes areanalogous to E-R entities


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Types of OperationsConstructor

Creates a new instance of a classQuery


UpdateAlters the state of an object

ScopeApplies to a full class rather than anindividual instance


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Representing Associations

Association: a relationship amonginstances of object classesAssociation role: the end of an


association where it connects to a classMultiplicity: indicates how many objectsparticipate in a give relationship


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UML associationsare analogous toE-R relationships.


mu t p c t esare analogous toE-R cardinalities.


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rolesmultiplicities


Multiplicity notation:0..10 means minimum of 0 and maximum of 101, 2 means can be either 1 or 2* means any number


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Association ClassAn association with its own attributes,operations, or relationships

UML


associationclasses areanalogousto E-Rassociative

entities.

D i d A ib A i i


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Derived Attributes, Associations,and Roles

Derived attributes are calculated

based on other attributes


Derived items are represented with a slash (/).


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Generalization

Superclass-subclass relationshipsSubclass inherits attributes, operations,and associations of the su erclass


Types of superclassesAbstract: cannot have any direct instancesConcrete: can have direct instances


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Generalization and inheritance implemented viasuperclass/subclasses in UML,supertypes/subtypes in E-R


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Polymorphic OperationsThe same operation may apply to two ormore classes in different waysAbstract operations


defined in abstract classesdefined the protocol, but not theimplementation of an operation

Methodsthe implementation of an operation


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Abstraction:Student is an abstract class and calc-


Polymorphism:Here, each type of student hasits own version of calc-tuition()

Class scope:tuitionPerCred is a class-wide attribute


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Aggregation and CompositionAggregation

A part-of relationship between acomponent and an aggregate object


CompositionAn aggregation in which the part objectbelongs to only one aggregate object and

lives and dies with the aggregate object


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Aggregation is represented withopen diamonds


Composition is represented withfilled diamonds


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SummaryIn this chapter you learned how to:

Define key data modeling terms.Draw entity-relationship (E-R) and class diagramsto represent common business situations.


analysis and design.Distinguish between unary, binary, and ternaryrelationships.Define four types of business rules.

Compare the capabilities of class diagrams vs. E-R diagrams.Relate data modeling to process and logicmodeling.

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