cs 4604: introducon to database management...

CS4604:Introduc0ontoDatabaseManagementSystems

B.AdityaPrakashLecture#7:En-ty/Rela-onal

Model---Part3

2

PurposeofE/RModel

§  TheE/Rmodelallowsustosketchthedesignofadatabaseinformally.–  Representdifferenttypesofdataandhowtheyrelatetoeachother

§  Designsaredrawingscalleden1ty-rela1onshipdiagrams.§  FairlymechanicalwaystoconvertE/Rdiagramstoreal

implementa-onslikerela-onaldatabasesexist.

Prakash2016 VTCS4604

Recap:Rela0onalModel

§  Builtaroundasingleconceptformodellingdata:therela-onortable.

§  Supportshigh-levelprogramminglanguage(SQL).

§  Hasanelegantmathema-caldesigntheory.§ MostcurrentDBMSarerela-onal.

Prakash2016 VTCS4604 3

Recap:TheRela0on

§  Arela-onisatwo-dimensionaltable:–  Rela-onßàtable.–  AXributeßàcolumnname.–  Tupleßàrow(nottheheaderrow).–  Databaseßàcollec-onofrela-ons.


Recap:TheSchema

§  Theschemaofarela-onisthenameoftherela-onfollowedbyaparanthe-sedlistofaXributes–  CoursesTaken(Student,Course,Grade)

§  Adesigninarela-onalmodelconsistsofasetofschemas.–  Suchasetofschemasiscalledarela-onaldatabaseschema


Conver0ngE/RDiagramstoRela0onalDesigns

§  En-tySetàRela-on– AXributeofEn-tySetàAXributeofaRela-on

§  Rela-onshipàrela-onwhoseaXributesare– AXributeoftherela-onshipitself–  KeyaXributesoftheconnecteden-tysets

§  Severalspecialcases:– Weaken-tysets.–  Combiningrela-ons(especiallyformany-onerela-onships)

–  ISArela-onshipsandsubclasses


ExampleforConversion


SchemasforNon-WeakEn0tySets

§  Foreachen-tyset,createarela-onwiththesamenameandwiththesamesetofaXributes

Students(Name,Address)Professors(Name,Office,Age)Departments(Name)


SchemasforWeakEn0tySets

§  Foreachweaken-tysetW,createarela-onwiththesamenamewhoseaXributesare:–  AXributesofW–  KeyaXributesofotheren-tysetsthathelpformthekeyforW

Courses(Number,DepartmentName,CourseName,Classroom,Enrollment)


SchemasforNon-Suppor0ngRela0onships

§  Foreachrela-onship,createarela-onwiththesamenamewhoseaXributesare–  AXributesoftherela-onshipitself.–  KeyaXributesoftheconnecteden-tysets(eveniftheyareweak)



§  Take



§  Take(StudentName,Address,Number,DepartmentName)§  Teach



§  Take(StudentName,Address,Number,DepartmentName)§  Teach(ProfessorName,Office,Number,DepartmentName)§  Evalua-on



§  Take(StudentName,Address,Number,DepartmentName)§  Teach(ProfessorName,Office,Number,DepartmentName)§  Evalua-on(StudentName,Address,ProfessorName,Office,

Number,DepartmentName,Grade)


RolesinRela0onships

§  Ifanen-tysetEappearsk>1-mesinarela-onshipR(indifferentroles),thekeyaXributesforEappeark-mesintherela-onforR,appropriatelyrenamed

PreReq(RequirerNumber,RequirerDeptName,RequirementNumber,RequirementDeptName)


CombiningRela0ons

§  Considermany-oneTeachrela-onshipfromCoursestoProfessors

§  Schemasare:Courses(Number,DepartmentName,CourseName,Classroom,Enrollment)Professors(Name,Office,Age)Teach(Number,DepartmentName,ProfessorName,Office)


CombiningRela0onsCourses(Number,DepartmentName,CourseName,Classroom,Enrollment)Professors(Name,Office,Age)Teach(Number,DepartmentName,ProfessorName,Office)

§  ThekeyforCoursesuniquelydeterminesallaXributesofTeach

§  Wecancombinetherela-onsforCoursesandTeachintoasinglerela-onwhoseaXributesare–  AlltheaXributesforCourses,–  AnyaXributesofTeach,and–  ThekeyaXributesofProfessors


RulesforCombiningRela0ons

§  Wecancombineintoonerela-onQ–  Therela-onforanen-tysetE–  allmany-to-onerela-onshipsR1,R2,…,RkfromEtootheren-tysetsE1,E2,…,Ekrespec-vely

§  TheaXributesofQare– AlltheaXributesofE– AnyaXributesofR1,R2,…,Rk–  ThekeyaXributesofE1,E2,…,Ek

§  WhatifRisamany-manyrela-onshipfromEtoF?


Suppor0ngRela0onships

§  SchemaforDepartmentsisDepartments(Name)§  SchemaforCoursesisCourses(Number,DepartmentName,CourseName,Classroom,Enrollment)

§ Whatistheschemaforoffer?


Suppor0ngRela0onships

§  Whatistheschemaforoffer?– Offer(Name,Number,DepartmentName)–  ButNameandDepartmentNameareiden-cal,sotheschemaforOfferisOffer(Number,DepartmentName)

–  TheschemaforOfferisasubsetoftheschemafortheweaken-tyset,sowecandispensewiththerela-onforOffer


SummaryofWeakEn0tySets

§  IfWisaweaken-tyset,therela-onforWhasaschemawhoseaXributesare–  allaXributesofW–  allaXributesofsuppor-ngrela-onshipsforW–  foreachsuppor-ngrela-onshipforWtoanen-tysetE–  thekeyaXributesofE

§  Thereisnorela-onforanysuppor-ngrela-onshipforWPrakash2016 VTCS4604 22

ISAtoRela0onal

§  Threeapproaches:– E/Rviewpoint– Object-orientedviewpoint– “FlaXen”viewpoint


RulesSa0sfiedbyanISAHierarchy

§  Thehierarchyhasarooten-tyset§  Therooten-tysethasakeythatiden-fieseveryen-tyrepresentedbythehierarchy

§  Apar-cularen-tycanhavecomponentsthatbelongtoen-tysetsofanysubtreeofthehierarchy,aslongasthatsubtreeincludestheroot


ExampleISAhierarchy


ISAtoRela0onalMethodI:E/RApproach

§  Createarela-onforeachen-tyset§  TheaXributesoftherela-onforanon-rooten-tysetEare–  theaXributesformingthekey(obtainedfromtheroot)and

–  anyaXributesofEitself§  Anen-tywithcomponentsinmul-pleen-tysetshastuplesinalltherela-onscorrespondingtotheseen-tysets

§  Donotcreatearela-onforanyisarela-onship§  Createarela-onforeveryotherrela-onshipPrakash2016 VTCS4604 26

Example:ISAtoRela0onalMethodI:E/RApproach

Students(ID,Name)Undergraduates(ID,Major)Graduates(ID,Major)Masters(ID,Thesis_-tle_MS)PhDs(ID,Thesis_-tle_PhD)UTA_for(ID,CourseNum,

DeptName)GTA_for(ID,CourseNum,

DeptName)


ISAtoRela0onalMethodII:“FlaOen”Approach

§  Createasinglerela-onfortheen-rehierarchy

§  AXributesare–  thekeyaXributesoftherootand–  theaXributesofeachen-tysetinthehierarchy

§  Handlerela-onshipsasbeforeStudents(ID,Name,UGMajor,GMajor,Thesis_-tle_MS,Thesis_-tle_PhD).


ISAtoRela0onalMethodIII:ObjectOrientedApproach

§  Treaten--esasobjectsbelongingtoasingleclass.§  “Class”==subtreeofthehierarchythatincludestheroot.

§  Enumerateallsubtreesofthehierarchythatcontaintheroot.

§  Foreachsuchsubtree,–  Createarela-onthatrepresentsen--esthathavecomponentsinexactlythatsubtree.

–  Theschemaforthisrela-onhasalltheaXributesofalltheen-tysetsinthatsubtree.

§  Schemaoftherela-onforarela-onshiphaskeyaXributesoftheconnecteden-tysets.


Example:ISAtoRela0onalMethodIII:ObjectOrientedApproach

Subtreesare:



Subtreesare:Students(ID,Name)



Subtreesare:Students(ID)StudentsUGs(ID,Major)



Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)



Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)StudentGsMasters(ID,Major,Thesis_-tle_MS)



Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)StudentGsMasters(ID,Major,Thesis_-tle_MS)StudentsGsPhDs(ID,Major,Thesis_-tle_PhD)



Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)StudentGsMasters(ID,Major,Thesis_-tle_MS)StudentsGsPhDs(ID,Major,Thesis_-tle_PhD)StudentsUGsGsMasters(ID,UGMajor,GradMajor,Thesis_-tle_MS)



Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)StudentGsMasters(ID,Major,Thesis_-tle_MS)StudentsGsPhDs(ID,Major,Thesis_-tle_PhD)StudentsUGsGsMasters(ID,UGMajor,GradMajor,Thesis_-tle_MS)


Whatothersubtreesexist?

ComparisonoftheThreeApproaches

§  Answeringqueries–  Itisexpensivetoanswerqueriesinvolvingseveralrela-ons

– QueriesaboutStudentsingeneral– Queriesaboutapar-cularsubclassofStudents



§  Numberofrela-onsfornrela-onsinthehierarchy– Weliketohaveasmallnumberofrela-ons– FlaXen

•  1– E/R

•  n– OO

•  Canbe2^n



§  Redundancyandspaceusage– FlaXen

•  MayhavealargenumberofNULLS

– E/R•  Severaltuplesperen-ty,butonlykeyaXributesarerepeated

– OO•  Onlyonetupleperen-ty


Inshort,goodenoughruleofthumb:usetheE/Rstyle

EXAMPLES


E/RExample1:USCongress(Handout2)

§  TheUSCongressisbicameralmeaningthatitiscomposedoftwohouses:theHouseofRepresenta-vesandtheSenate.EverystatehasexactlytwoSenators(ajuniorandaseniormember),butavariablenumberofRepresenta-ves(exactlyoneperdistrict).Nosenatorcanrepresentmorethanonestateata-me.Likewise,noRepresenta-vecanservemorethanonedistrictata-me.Everystatehasavariablenumberofdistricts(dependentonpopula-on),buteverystatehasatleastonedistrict(inastatelikeDelawarethedistrictboundariesarethestate'sborders).Districtshavenumbers(e.g.,district1).AgivenCongressperson(SenatororRepresenta-ve)cannotserveinbothhousesatagiven-me.Congresspeoplehavenamesande-mailaddresses.EveryCongresspersonisamemberofexactlyonepoli-calparty.ExactlyonememberoftheHouseisdesignatedasSpeakeroftheHouse.Lastly,CongresspeoplebelongtoCongressionalcommiXeeswhichhavenamesandsponsorbills,whichalsohavenames."



Notes:1.  SpeakerisNOTNULL

E/RExample2:CompanyDB§  A company database needs to store informa-on about

employees (iden-fied by ssn, with salary and phone asa X r i b u t e s ) , d e p a r tmen t s ( i d e n -fi e d b y d n o ,with dname and budget as aXributes), and children ofemployees (with name and age as aXr ibutes) .Employees work in departments; each departmentis managed by an employee; a child must be iden-fieduniquely by name when the parent (who is an employee;assume that only one parent works for the company) isknown.


§  A company database needs to store informa-on about employees(iden-fied by ssn, with salary and phone as aXributes), departments(iden-fiedbydno,withdnameandbudgetasaXributes),andchildrenofemployees(withnameandageasaXributes).

Note:Childisaweaken-ty


§  Employeesworkindepartments;§  eachdepartmentismanagedbyanemployee;


§  achildmustbe iden-fieduniquelybynamewhentheparent (who isanemployee;assumethatonlyoneparentworksforthecompany)isknown.


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