cs 4604: introduction to database management...
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CS4604:IntroductiontoDatabaseManagementSystems
B.AdityaPrakashLecture#7:Entity/Relational
Model---Part3
2
PurposeofE/RModel
§ TheE/Rmodelallowsustosketchthedesignofadatabaseinformally.– Representdifferenttypesofdataandhowtheyrelatetoeachother
§ Designsaredrawingscalledentity-relationshipdiagrams.§ FairlymechanicalwaystoconvertE/Rdiagramstoreal
implementationslikerelationaldatabasesexist.
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Recap:RelationalModel
§ Builtaroundasingleconceptformodellingdata:therelationortable.
§ Supportshigh-levelprogramminglanguage(SQL).
§ Hasanelegantmathematicaldesigntheory.§ MostcurrentDBMSarerelational.
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Recap:TheRelation
§ Arelationisatwo-dimensionaltable:– Relationßàtable.– Attributeßàcolumnname.– Tupleßàrow(nottheheaderrow).– Databaseßàcollectionofrelations.
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Recap:TheSchema
§ Theschemaofarelationisthenameoftherelationfollowedbyaparanthetisedlistofattributes– CoursesTaken(Student,Course,Grade)
§ Adesigninarelationalmodelconsistsofasetofschemas.– Suchasetofschemasiscalledarelationaldatabaseschema
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ConvertingE/RDiagramstoRelationalDesigns
§ EntitySetàRelation– AttributeofEntitySetàAttributeofaRelation
§ Relationshipàrelationwhoseattributesare– Attributeoftherelationshipitself– Keyattributesoftheconnectedentitysets
§ Severalspecialcases:– Weakentitysets.– Combiningrelations(especiallyformany-onerelationships)
– ISArelationshipsandsubclasses
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ExampleforConversion
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SchemasforNon-WeakEntitySets
§ Foreachentityset,createarelationwiththesamenameandwiththesamesetofattributes
Students(Name,Address)Professors(Name,Office,Age)Departments(Name)
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SchemasforWeakEntitySets
§ ForeachweakentitysetW,createarelationwiththesamenamewhoseattributesare:– AttributesofW– KeyattributesofotherentitysetsthathelpformthekeyforW
Courses(Number,DepartmentName,CourseName,Classroom,Enrollment)
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SchemasforNon-SupportingRelationships
§ Foreachrelationship,createarelationwiththesamenamewhoseattributesare– Attributesoftherelationshipitself.– Keyattributesoftheconnectedentitysets(eveniftheyareweak)
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SchemasforNon-SupportingRelationships
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SchemasforNon-SupportingRelationships
§ Take
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SchemasforNon-SupportingRelationships
§ Take(StudentName,Address,Number,DepartmentName)§ Teach
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SchemasforNon-SupportingRelationships
§ Take(StudentName,Address,Number,DepartmentName)§ Teach(ProfessorName,Office,Number,DepartmentName)§ Evaluation
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SchemasforNon-SupportingRelationships
§ Take(StudentName,Address,Number,DepartmentName)§ Teach(ProfessorName,Office,Number,DepartmentName)§ Evaluation(StudentName,Address,ProfessorName,Office,
Number,DepartmentName,Grade)
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RolesinRelationships
§ IfanentitysetEappearsk>1timesinarelationshipR(indifferentroles),thekeyattributesforEappearktimesintherelationforR,appropriatelyrenamed
PreReq(RequirerNumber,RequirerDeptName,RequirementNumber,RequirementDeptName)
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CombiningRelations
§ Considermany-oneTeachrelationshipfromCoursestoProfessors
§ Schemasare:Courses(Number,DepartmentName,CourseName,Classroom,Enrollment)Professors(Name,Office,Age)Teach(Number,DepartmentName,ProfessorName,Office)
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CombiningRelationsCourses(Number,DepartmentName,CourseName,Classroom,Enrollment)Professors(Name,Office,Age)Teach(Number,DepartmentName,ProfessorName,Office)
§ ThekeyforCoursesuniquelydeterminesallattributesofTeach
§ WecancombinetherelationsforCoursesandTeachintoasinglerelationwhoseattributesare– AlltheattributesforCourses,– AnyattributesofTeach,and– ThekeyattributesofProfessors
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RulesforCombiningRelations
§ WecancombineintoonerelationQ– TherelationforanentitysetE– allmany-to-onerelationshipsR1,R2,…,RkfromEtootherentitysetsE1,E2,…,Ekrespectively
§ TheattributesofQare– AlltheattributesofE– AnyattributesofR1,R2,…,Rk– ThekeyattributesofE1,E2,…,Ek
§ WhatifRisamany-manyrelationshipfromEtoF?
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SupportingRelationships
§ SchemaforDepartmentsisDepartments(Name)§ SchemaforCoursesisCourses(Number,DepartmentName,CourseName,Classroom,Enrollment)
§ Whatistheschemaforoffer?
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SupportingRelationships
§ Whatistheschemaforoffer?– Offer(Name,Number,DepartmentName)– ButNameandDepartmentNameareidentical,sotheschemaforOfferisOffer(Number,DepartmentName)
– TheschemaforOfferisasubsetoftheschemafortheweakentityset,sowecandispensewiththerelationforOffer
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SummaryofWeakEntitySets
§ IfWisaweakentityset,therelationforWhasaschemawhoseattributesare– allattributesofW– allattributesofsupportingrelationshipsforW– foreachsupportingrelationshipforWtoanentitysetE– thekeyattributesofE
§ ThereisnorelationforanysupportingrelationshipforWPrakash2018 VTCS4604 22
ISAtoRelational
§ Threeapproaches:– E/Rviewpoint– Object-orientedviewpoint– “Flatten”viewpoint
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RulesSatisfiedbyanISAHierarchy
§ Thehierarchyhasarootentityset§ Therootentitysethasakeythatidentifieseveryentityrepresentedbythehierarchy
§ Aparticularentitycanhavecomponentsthatbelongtoentitysetsofanysubtreeofthehierarchy,aslongasthatsubtreeincludestheroot
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ExampleISAhierarchy
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ISAtoRelationalMethodI:E/RApproach
§ Createarelationforeachentityset§ Theattributesoftherelationforanon-rootentitysetEare– theattributesformingthekey(obtainedfromtheroot)and
– anyattributesofEitself§ Anentitywithcomponentsinmultipleentitysetshastuplesinalltherelationscorrespondingtotheseentitysets
§ Donotcreatearelationforanyisarelationship§ CreatearelationforeveryotherrelationshipPrakash2018 VTCS4604 26
Example:ISAtoRelationalMethodI:E/RApproach
Students(ID,Name)Undergraduates(ID,Major)Graduates(ID,Major)Masters(ID,Thesis_title_MS)PhDs(ID,Thesis_title_PhD)UTA_for(ID,CourseNum,
DeptName)GTA_for(ID,CourseNum,
DeptName)
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ISAtoRelationalMethodII:“Flatten”Approach
§ Createasinglerelationfortheentirehierarchy
§ Attributesare– thekeyattributesoftherootand– theattributesofeachentitysetinthehierarchy
§ HandlerelationshipsasbeforeStudents(ID,Name,UGMajor,GMajor,Thesis_title_MS,Thesis_title_PhD).
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ISAtoRelationalMethodIII:ObjectOrientedApproach
§ Treatentitiesasobjectsbelongingtoasingleclass.§ “Class”==subtreeofthehierarchythatincludestheroot.
§ Enumerateallsubtreesofthehierarchythatcontaintheroot.
§ Foreachsuchsubtree,– Createarelationthatrepresentsentitiesthathavecomponentsinexactlythatsubtree.
– Theschemaforthisrelationhasalltheattributesofalltheentitysetsinthatsubtree.
§ Schemaoftherelationforarelationshiphaskeyattributesoftheconnectedentitysets.
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Example:ISAtoRelationalMethodIII:ObjectOrientedApproach
Subtreesare:
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Example:ISAtoRelationalMethodIII:ObjectOrientedApproach
Subtreesare:Students(ID,Name)
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Example:ISAtoRelationalMethodIII:ObjectOrientedApproach
Subtreesare:Students(ID)StudentsUGs(ID,Major)
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Example:ISAtoRelationalMethodIII:ObjectOrientedApproach
Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)
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Example:ISAtoRelationalMethodIII:ObjectOrientedApproach
Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)StudentGsMasters(ID,Major,Thesis_title_MS)
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Example:ISAtoRelationalMethodIII:ObjectOrientedApproach
Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)StudentGsMasters(ID,Major,Thesis_title_MS)StudentsGsPhDs(ID,Major,Thesis_title_PhD)
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Example:ISAtoRelationalMethodIII:ObjectOrientedApproach
Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)StudentGsMasters(ID,Major,Thesis_title_MS)StudentsGsPhDs(ID,Major,Thesis_title_PhD)StudentsUGsGsMasters(ID,UGMajor,GradMajor,Thesis_title_MS)
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Example:ISAtoRelationalMethodIII:ObjectOrientedApproach
Subtreesare:Students(ID)StudentsUGs(ID,Major)StudentGs(ID,Major)StudentGsMasters(ID,Major,Thesis_title_MS)StudentsGsPhDs(ID,Major,Thesis_title_PhD)StudentsUGsGsMasters(ID,UGMajor,GradMajor,Thesis_title_MS)
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Whatothersubtreesexist?
ComparisonoftheThreeApproaches
§ Answeringqueries– Itisexpensivetoanswerqueriesinvolvingseveralrelations
– QueriesaboutStudentsingeneral– QueriesaboutaparticularsubclassofStudents
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ComparisonoftheThreeApproaches
§ Numberofrelationsfornrelationsinthehierarchy– Weliketohaveasmallnumberofrelations– Flatten
• 1– E/R
• n– OO
• Canbe2^n
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ComparisonoftheThreeApproaches
§ Redundancyandspaceusage– Flatten
• MayhavealargenumberofNULLS
– E/R• Severaltuplesperentity,butonlykeyattributesarerepeated
– OO• Onlyonetupleperentity
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Inshort,goodenoughruleofthumb:usetheE/Rstyle
EXAMPLES
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E/RExample1:USCongress(Handout2)
§ TheUSCongressisbicameralmeaningthatitiscomposedoftwohouses:theHouseofRepresentativesandtheSenate.EverystatehasexactlytwoSenators(ajuniorandaseniormember),butavariablenumberofRepresentatives(exactlyoneperdistrict).Nosenatorcanrepresentmorethanonestateatatime.Likewise,noRepresentativecanservemorethanonedistrictatatime.Everystatehasavariablenumberofdistricts(dependentonpopulation),buteverystatehasatleastonedistrict(inastatelikeDelawarethedistrictboundariesarethestate'sborders).Districtshavenumbers(e.g.,district1).AgivenCongressperson(SenatororRepresentative)cannotserveinbothhousesatagiventime.Congresspeoplehavenamesande-mailaddresses.EveryCongresspersonisamemberofexactlyonepoliticalparty.ExactlyonememberoftheHouseisdesignatedasSpeakeroftheHouse.Lastly,CongresspeoplebelongtoCongressionalcommitteeswhichhavenamesandsponsorbills,whichalsohavenames."
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Notes:1. SpeakerisNOTNULL
E/RExample2:CompanyDB§ A company database needs to store information about
employees (identified by ssn, with salary and phone asa t t r i b u t e s ) , d epa r tmen t s ( i d en t i f i e d b y dno ,with dname and budget as attributes), and children ofemployees (with name and age as attr ibutes) .Employees work in departments; each departmentis managed by an employee; a child must be identifieduniquely by name when the parent (who is an employee;assume that only one parent works for the company) isknown.
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§ A company database needs to store information about employees(identified by ssn, with salary and phone as attributes), departments(identifiedbydno,withdnameandbudgetasattributes),andchildrenofemployees(withnameandageasattributes).
Note:Childisaweakentity
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§ Employeesworkindepartments;§ eachdepartmentismanagedbyanemployee;
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§ achildmustbe identifieduniquelybynamewhentheparent (who isanemployee;assumethatonlyoneparentworksforthecompany)isknown.
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