DatabaseDesign

AnnaMonreale

Needtodesign

•  Databaseareo.enborndesignless,fromahugespreadsheet

•  Anomaliesarise,becauseofredundancy

•  Redundancygenerateerrors

•  Designmustinvolvetheuser

Anomalies

PhasesforDBrealiza7on•  Userrequirementsanalysis&specifica7on

–  collec<nguserneedsandnormalizingthemaccordingtostandards

•  Conceptualdesign–  isthephaseinwhichrequirementsareformalizedandintegratedinto

aglobalconceptualschema–  usingaDBMS-independentconceptuallanguage

•  Logicaldesign–  theconceptualschemaismappedintoalogicalschemausingthedata

modelsupportedbytheDBMSchosenfortheimplementa<on

•  Physicaldesign–  concernstheselec<onofthedatastructuresusedtostoreand

retrievethedata.

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UserRequirementsAnalysis

•  Hardac<vitybecausehardtostandardize•  Sugges<ons

–  Involvetheusersmany<mesforcon<nuouschecks–  Considerthepointofviewoftheapplica<onsusers–  Tobesurethatyouareusingacommonlanguage–  Iden<fycasestudiesthatyoucandiscussindetailstoiden<fytheproper<estobecapturedbythemodel

Mod. Concettuale >> Concetti Fondamentali >> Introduzione

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AnObjectOrientedLanguagefordatadesign

•  Realiza<onofadiagramrepresen<ngtheconceptualmodelofthedatabase

•  Components:–  Classes(collec<ons)

–  Rela7onshipsamongclasses

–  Sub-collec7onslinks

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ClassDiagram

•  PhaseofAnalasys– Needtoadopttherightlevelofabstrac<on

•  Inpar<cular– Wedon’tneedallaPributes– TypeofaPributesisnotnecessary

Mod. Concettuale >> Concetti Fondamentali >> Il Diagramma delle Classi

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Example:UniversityDB•  Weneedtodesignthedatabaseformanagingdataaboutcoursesof

computersciencedegreeattheUniversityofPisa

•  Thesystemmustmanagedataaboutstudentsofthemasterprogramandbachelorprogram.Foreachoneweneedtomaintaindatarelatedtothestudentsexams.

•  Weneedtorecorddataaboutcoursesandthestudentsexamsforeachcourse.

•  Foreachcoursewewanttorecordteachers,whomaybemorethanone.Moreoverateachermaybeinternalorexternal.

•  Foreachteacherwehaveoneormorephonenumbers.

•  Foreachstudentweneedtorecordthesupervisor(ateacher).BachelorstudentsmayaskasupervisoronlywhentheyareaPendingthethirdyear.

•  Lastlythesystemmustmaintaininforma<onaboutthetutoringac<vi<esofmasterstudents,thathelpbachelorstudents.

Mod. Concettuale >> Concetti Fondamentali >> Il Diagramma delle Classi

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Classes

•  “Concepts”oftherealitytobemodelled–  facts,people,things,–  examples:student,course,exam,teacher

•  Instancesofaclass–  en<<es,objectsoftherealitytobemodelled

•  ClasseshaveaNributes–  Proper<esrelevantfortheapplica<on

ClasswithaNributes

•  Apersonclass,withaPributes:– Name– SSN(key)– Address

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Classes

•  Instancesoftheclasses

Students

studentID

surname

name

year

{77777, Bruno Pasquale, 1}

{444, Pinco, Palla, 3} {88888,

Pinco, Pietro, 1}

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Rela7onship

•  Rela7onshipbetweenclasses•  Logiclinkrelevantfortheapplica<on•  ex:teachingbetweenteacherandcourse•  ex:studentpassesanexam

•  Instanceofarela7onship•  Asetofedgesbetweeninstancesbelongingtotheinvolvedclasses

Mod. Concettuale >> Concetti Fondamentali >> Il Diagramma delle Classi

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Rela7onship:Instances

{Dino, Pedreschi, Full}

{77777, Bruno Pasquale, 1}

{444, Pinco, Palla, 3}

{88888, Pinco, Pietro, 1}

{Anna, Monreale, Associate}

Teachers

Students

{111, Rossi, Mario 1}

{Giannotti, Fosca}

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Cardinality

•  Constraintsonrela7onships– Constraintsonthenumberofedgesbetweeninstancesofclasses

•  MinimalCardinality– 0or1

•  MaximalCardinality– 1ormany

Students Exams

Students Teachers

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Cardinality

{Pedreschi, Dino, ordinario}

{77777, Bruno Pasquale, 1}

{444, Pinco, Palla, 3}

{88888, Pinco, Pietro, 1}

{Monreale, Anna, associato}

Teachers

Students

{111, Rossi, Mario 1}

Giannotti Fosca}

Supervisor >

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Cardinality(upperbound)

•  Classifica<onoftherela<onshipswrtthecardinality– OnetoOne:maximalcardinalityequalto1forbothclasses

•  Manages[Managers,Departments]– OnetoMany:maximalcardinalityequalto1foraclassandmany(N)fortheotherone

•  Owns[Persons,Cars]– ManytoMany:maximalcardinalityequaltoNforbothclasses

•  Teaching[Course,Teacher]

Cardinality(lowerbound)•  Sixteencombin<ons:

– Onetomanytotal/par<al– Onetomanypar<al/par<al

– Onetomanypar<al/total

– Onetomanytotal/total

ExamsStudents

ExamsStudents

ExamsStudents

ExamsStudents

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ClassHierarchy

•  Asubclass:– asubsetofclasselements,forwhichweplantocollectmoreinforma<on:

– ex:StudentsissubclassofPersons– ex:Internalandexternalteachersaresubclassesofthegenericconcept“teacher”

Mod. Concettuale >> Concetti Fondamentali >> Il Diagramma delle Classi

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ClassHierarchy

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Notes

•  Some<meitisnecesarytoaddnotesinthediagramtoexpresssomeconstraints–  Ex:BachelorstudentsmayaskasupervisoronlywhentheyareaPendingthethirdyear.

Students Teachers

Bachelor students may have a supervisor only during the third year

Rela7onshpwithaNributes•  Some<mesarela<onshipmayhavesomeproper<esthat

characterizeeachinstanceoftherela<onship•  “Johnisoccupyingtheroom105atLeMeridien-Houston,ata$145rate”

•  Thisisarela<onshipinstancebetweenpersonsandrooms,witharateaPribute

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RecursiveRela7onships

{a01, Anna, Monreale}

Persons

{a02, Sofia, Giuliani}

{p01, Dino, Palla }

{p02, Paolo, Palla}

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TernaryRela7onship

•  Ternaryfactsexistalso•  “JohnbookedflightFK354/13-6-2000withY2fare”

Keepitsimple

•  Wheneveritmakessense,upgradearela7onshipwithaNributes,oraternaryone,toacollec<on

FromANributestoclasses

Fromternarytonewclass