数据库理论及基础面试题.docx
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数据库理论及基础面试题.docx
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数据库理论及基础面试题
数据库理论及基础面试题
问题:
数据库理论及基础面试题回答:
1.WhatisdatabaseAdatabaseisalogicallycoherentcollectionofdatawithsomeinherentmeaning,representingsomeaspectofrealworldandwhichisdesigned,builtandpopulatedwithdataforaspecificpurpose.
2.WhatisDBMSItisacollectionofprogramsthatenablesusertocreateandmaintainadatabase.Inotherwordsitisgeneral-purposesoftwarethatprovidestheuserswiththeprocessesofdefining,constructingandmanipulatingthedatabaseforvariousapplications.
3.WhatisaDatabasesystemThedatabaseandDBMSsoftwaretogetheriscalledasDatabasesystem.
4.AdvantagesofDBMSRedundancyiscontrolled.Unauthorisedaccessisrestricted.Providingmultipleuserinterfaces.Enforcingintegrityconstraints.Providingbackupandrecovery.
5.DisadvantageinFileProcessingSystemDataredundancyinconsistency.Difficultinaccessingdata.Dataisolation.Dataintegrity.Concurrentaccessisnotpossible.SecurityProblems.
6.DescribethethreelevelsofdataabstractionThearethreelevelsofabstraction:
Physicallevel:
Thelowestlevelofabstractiondescribeshowdataarestored.Logicallevel:
Thenexthigherlevelofabstraction,describeswhatdataarestoredindatabaseandwhatrelationshipamongthosedata.Viewlevel:
Thehighestlevelofabstractiondescribesonlypartofentiredatabase.7.DefinetheintegrityrulesTherearetwoIntegrityrules.
EntityIntegrity:
Statesthat“PrimarykeycannothaveNULLvalue”ReferentialIntegrity:
Statesthat“ForeignKeycanbeeitheraNULLvalueorshouldbePrimaryKeyvalueofotherrelation.
8.WhatisextensionandintensionExtension-
Itisthenumberoftuplespresentinatableatanyinstance.Thisistimedependent.
Intension-
Itisaconstantvaluethatgivesthename,structureoftableandtheconstraintslaidonit.
9.WhatisSystemRWhatareitstwomajorsubsystemsSystemRwasdesignedanddevelopedoveraperiodof1974-79atIBMSanJoseResearchCenter.ItisaprototypeanditspurposewastodemonstratethatitispossibletobuildaRelationalSystemthatcanbeusedinareallifeenvironmenttosolvereallifeproblems,withperformanceatleastcomparabletothatofexistingsystem.
Itstwosubsystemsare
ResearchStorageSystemRelationalDataSystem.
10.HowisthedatastructureofSystemRdifferentfromtherelationalstructureUnlikeRelationalsystemsinSystemR
DomainsarenotsupportedEnforcementofcandidatekeyuniquenessisoptionalEnforcementofentityintegrityisoptionalReferentialintegrityisnotenforced
11.WhatisDataIndependenceDataindependencemeansthat“theapplicationisindependentofthestoragestructureandaccessstrategyofdata”.Inotherwords,Theabilitytomodifytheschemadefinitioninonelevelshouldnotaffecttheschemadefinitioninthenexthigherlevel.
TwotypesofDataIndependence:
PhysicalDataIndependence:
Modificationinphysicallevelshouldnotaffectthelogicallevel.LogicalDataIndependence:
Modificationinlogicallevelshouldaffecttheviewlevel.NOTE:
LogicalDataIndependenceismoredifficulttoachieve
12.WhatisaviewHowitisrelatedtodataindependenceAviewmaybethoughtofasavirtualtable,thatis,atablethatdoesnotreallyexistinitsownrightbutisinsteadderivedfromoneormoreunderlyingbasetable.Inotherwords,thereisnostoredfilethatdirectrepresentstheviewinsteadadefinitionofviewisstoredindatadictionary.
Growthandrestructuringofbasetablesisnotreflectedinviews.Thustheviewcaninsulateusersfromtheeffectsofrestructuringandgrowthinthedatabase.Henceaccountsforlogicaldataindependence.
13.WhatisDataModelAcollectionofconceptualtoolsfordescribingdata,datarelationshipsdatasemanticsandconstraints.
14.WhatisE-RmodelThisdatamodelisbasedonrealworldthatconsistsofbasicobjectscalledentitiesandofrelationshipamongtheseobjects.Entitiesaredescribedinadatabasebyasetofattributes.
15.WhatisObjectOrientedmodelThismodelisbasedoncollectionofobjects.Anobjectcontainsvaluesstoredininstancevariableswithintheobject.Anobjectalsocontainsbodiesofcodethatoperateontheobject.Thesebodiesofcodearecalledmethods.Objectsthatcontainsametypesofvaluesandthesamemethodsaregroupedtogetherintoclasses.
16.WhatisanEntityItisathingintherealworldwithanindependentexistence.
17.WhatisanEntitytypeItisacollection(set)ofentitiesthathavesameattributes.
18.WhatisanEntitysetItisacollectionofallentitiesofparticularentitytypeinthedatabase.
19.WhatisanExtensionofentitytypeThecollectionsofentitiesofaparticularentitytypearegroupedtogetherintoanentityset.
20.WhatisWeakEntitysetAnentitysetmaynothavesufficientattributestoformaprimarykey,anditsprimarykeycompromisesofitspartialkeyandprimarykeyofitsparententity,thenitissaidtobeWeakEntityset.
21.WhatisanattributeItisaparticularproperty,whichdescribestheentity.
22.WhatisaRelationSchemaandaRelationArelationSchemadenotedbyR(A1,A2,…,An)ismadeupoftherelationnameRandthelistofattributesAithatitcontains.Arelationisdefinedasasetoftuples.Letrbetherelationwhichcontainssettuples(t1,t2,t3,,tn).Eachtupleisanorderedlistofn-valuest=(v1,v2,,vn).
23.WhatisdegreeofaRelationItisthenumberofattributeofitsrelationschema.
24.WhatisRelationshipItisanassociationamongtwoormoreentities.
25.WhatisRelationshipsetThecollection(orset)ofsimilarrelationships.
26.WhatisRelationshiptypeRelationshiptypedefinesasetofassociationsorarelationshipsetamongagivensetofentitytypes.
27.WhatisdegreeofRelationshiptypeItisthenumberofentitytypeparticipating.
25.WhatisDDL(DataDefinitionLanguage)AdatabaseschemaisspecifiesbyasetofdefinitionsexpressedbyaspeciallanguagecalledDDL.
26.WhatisVDL(ViewDefinitionLanguage)Itspecifiesuserviewsandtheirmappingstotheconceptualschema.
27.WhatisSDL(StorageDefinitionLanguage)Thislanguageistospecifytheinternalschema.Thislanguagemayspecifythemappingbetweentwoschemas.
28.WhatisDataStorageDefinitionLanguageThestoragestructuresandaccessmethodsusedbydatabasesystemarespecifiedbyasetofdefinitioninaspecialtypeofDDLcalleddatastorage-definitionlanguage.
29.WhatisDML(DataManipulationLanguage)Thislanguagethatenableusertoaccessormanipulatedataasorganisedbyappropriatedatamodel.
ProceduralDMLorLowlevel:
DMLrequiresausertospecifywhatdataareneededandhowtogetthosedata.Non-ProceduralDMLorHighlevel:
DMLrequiresausertospecifywhatdataareneededwithoutspecifyinghowtogetthosedata.
31.WhatisDMLCompilerIttranslatesDMLstatementsinaquerylanguageintolow-levelinstructionthatthequeryevaluationenginecanunderstand.
32.WhatisQueryevaluationengineItexecuteslow-levelinstructiongeneratedbycompiler.
33.WhatisDDLInterpreterItinterpretsDDLstatementsandrecordthemintablescontainingmetadata.
34.WhatisRecord-at-a-timeTheLowlevelorProceduralDMLcanspecifyandretrieveeachrecordfromasetofrecords.ThisretrieveofarecordissaidtobeRecord-at-a-time.
35.WhatisSet-at-a-timeorSet-orientedTheHighlevelorNon-proceduralDMLcanspecifyandretrievemanyrecordsinasingleDMLstatement.ThisretrieveofarecordissaidtobeSet-at-a-timeorSet-oriented.
36.WhatisRelationalAlgebraItisproceduralquerylanguage.Itconsistsofasetofoperationsthattakeoneortworelationsasinputandproduceanewrelation.
37.WhatisRelationalCalculusItisanappliedpredicatecalculusspecificallytailoredforrelationaldatabasesproposedbyE.F.Codd.E.g.oflanguagesbasedonitareDSLALPHA,QUEL.
38.HowdoesTuple-orientedrelationalcalculusdifferfromdomain-orientedrelationalcalculusThetuple-orientedcalculususesatuplevariablesi.e.,variablewhoseonlypermittedvaluesaretuplesofthatrelation.E.g.QUEL
Thedomain-orientedcalculushasdomainvariablesi.e.,variablesthatrangeovertheunderlyingdomainsinsteadofoverrelation.E.g.ILL,DEDUCE.
39.WhatisnormalizationItisaprocessofanalysingthegivenrelationschemasbasedontheirFunctionalDependencies(FDs)andprimarykeytoachievetheproperties
MinimizingredundancyMinimizinginsertion,deletionandupdateanomalies.
40.WhatisFunctionalDependencyAFunctionaldependencyisdenotedbyXYbetweentwosetsofattributesXandYthataresubsetsofRspecifiesaconstraintonthepossibletuplethatcanformarelationstaterofR.Theconstraintisforanytwotuplest1andt2inrift1[X]=t2[X]thentheyhavet1[Y]=t2[Y].ThismeansthevalueofXcomponentofatupleuniquelydeterminesthevalueofcomponentY.
41.WhenisafunctionaldependencyFsaidtobeminimalEverydependencyinFhasasingleattributeforitsrighthandside.WecannotreplaceanydependencyXAinFwithadependencyYAwhereYisapropersubsetofXandstillhaveasetofdependencythatisequivalenttoF.WecannotremoveanydependencyfromFandstillhavesetofdependencythatisequivalenttoF.
42.WhatisMultivalueddependencyMultivalueddependencydenotedbyXYspecifiedonrelationschemaR,whereXandYarebothsubsetsofR,specifiesthefollowingconstraintonanyrelationrofR:
iftwotuplest1andt2existinrsuchthatt1[X]=t2[X]thent3andt4shouldalsoexistinrwiththefollowingproperties
t3[x]=t4[X]=t1[X]=t2[X]t3[Y]=t1[Y]andt4[Y]=t2[Y]t3[Z]=t2[Z]andt4[Z]=t1[Z]where[Z=(R-(XUY))]
43.WhatisLossles
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