数据库理论及基础面试题.docx

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.WhatisDBMSItisacollectionofprogramsthatenables

usertocreateandmaintainadatabase.Inotherwordsitisgeneral-purposesoftwarethatprovidestheuserswiththeprocessesofdefining,constructingandmanipulatingthedatabaseforvariousapplications.

3.WhatisaDatabasesystemThedatabaseandDBMSsoftwaretogetheriscalledasDatabasesystem.

4.AdvantagesofDBMSRedundancyis

controlled.Unauthorisedaccessisrestricted.Providingmultiple

userinterfaces.Enforcingintegrityconstraints.Providing

backupandrecovery.

5.DisadvantageinFileProcessingSystemData

redundancyinconsistency.Difficultinaccessingdata.Data

isolation.Dataintegrity.Concurrentaccessisnotpossible.SecurityProblems.

6.DescribethethreelevelsofdataabstractionTheare

threelevelsofabstraction:

Physicallevel:

Thelowestlevelofabstractiondescribeshowdataarestored.Logicallevel:

Thenexthigherlevelofabstraction,describeswhatdataarestoredindatabaseandwhatrelationshipamongthosedata.Viewlevel:

Thehighestlevelofabstractiondescribesonlypartofentiredatabase.7.DefinetheintegrityrulesTherearetwoIntegrityrules.

EntityIntegrity:

Statesthat“PrimarykeycannothaveNULLvalue”ReferentialIntegrity:

Statesthat“ForeignKeycan

beeitheraNULLvalueorshouldbePrimaryKeyvalueofotherrelation.

8.WhatisextensionandintensionExtension-

Itisthenumberoftuplespresentinatableatanyinstanee.Thisistimedependent.

Intension-

Itisaconstantvaluethatgivesthename,structureoftableandtheconstraintslaidonit.

9.WhatisSystemRWhatareitstwomajorsubsystemsSystemRwasdesignedanddevelopedoveraperiodof1974-79atIBMSanJoseResearchCenter.ItisaprototypeanditspurposewastodemonstratethatitispossibletobuildaRelationalSystemthatcanbeusedinareallifeenvironmenttosolvereallifeproblems,withperformaneeatleastcomparabletothatofexistingsystem.

Itstwosubsystemsare

ResearchStorageSystemRelationalDataSystem.

10.HowisthedatastructureofSystemRdifferentfrom

therelationalstructureUnlikeRelationalsystemsinSystemR

DomainsarenotsupportedEnforcementofcandidatekeyuniquenessisoptionalEnforcementofentityintegrityis

optionalReferentialintegrityisnotenforced

11.WhatisDataIndependenceDataindependence

meansthat“theapplicationisindependentofthestorage

structureandaccessstrategyofdata”.Inotherwords,Theabilitytomodifytheschemadefinitioninonelevelshouldnot

affecttheschemadefinitioninthenexthigherlevel.

TwotypesofDataIndependence:

PhysicalDataIndependence:

Modificationinphysicallevelshouldnotaffectthelogicallevel.LogicalDataIndependence:

Modificationinlogicallevelshouldaffecttheviewlevel.NOTE:

LogicalDataIndependenceismoredifficulttoachieve

12.WhatisaviewHowitisrelatedtodataindependence

Aviewmaybethoughtofasavirtualtable,thatis,atablethatdoesnotreallyexistinitsownrightbutisinsteadderivedfromoneormoreunderlyingbasetable.Inotherwords,thereisnostoredfilethatdirectrepresentstheviewinsteadadefinitionofviewisstoredindatadictionary.

Growthandrestructuringofbasetablesisnotreflectedin

views.Thustheviewcaninsulateusersfromtheeffectsofrestructuringandgrowthinthedatabase.Henceaccountsforlogicaldataindependence.

13.WhatisDataModelAcollectionofconceptualtoolsfordescribingdata,datarelationshipsdatasemanticsandconstraints.

14.WhatisE-RmodelThisdatamodelisbasedonrealworldthatconsistsofbasicobjectscalledentitiesandofrelationshipamongtheseobjects.Entitiesaredescribedinadatabasebyasetofattributes.

15.WhatisObjectOrientedmodelThismodelisbasedon

collectionofobjects.Anobjectcontainsvaluesstoredininstaneevariableswithintheobject.Anobjectalsocontainsbodiesofcodethatoperateontheobject.Thesebodiesofcodearecalledmethods.Objectsthatcontainsametypesofvaluesandthesamemethodsaregroupedtogetherintoclasses.

16.WhatisanEntityItisathingintherealworldwithan

independentexistenee.

17.WhatisanEntitytypeItisaeolleetion（set）ofentities

thathavesameattributes.

18.WhatisanEntitysetItisaeolleetionofallentitiesofparticularentitytypeinthedatabase.

19.WhatisanExtensionofentitytypeTheeolleetionsofentitiesofapartieularentitytypearegroupedtogetherintoanentityset.

20.WhatisWeakEntitysetAnentitysetmaynothavesuffieientattributestoformaprimarykey,anditsprimarykeyeompromisesofitspartialkeyandprimarykeyofitsparententity,thenitissaidtobeWeakEntityset.

21.WhatisanattributeItisapartieularproperty,whiehdeseribestheentity.

22.WhatisaRelationSehemaandaRelationArelationSehemadenotedbyR（A1,A2,…，An）ismadeupoftherelationnameRandthelistofattributesAithatitcontains.A

relationisdefinedasasetoftuples.Letrbetherelationwhiehcontainssettuples（t1,t2,t3,,tn）.Eaehtupleisanorderedlistofn-valuest=（v1,v2,,vn）.

23.WhatisdegreeofaRelationItisthenumberofattributeofitsrelationschema.

24.WhatisRelationshipItisanassociationamongtwoormoreentities.

25.WhatisRelationshipsetThecollection（orset）of

similarrelationships.

26.WhatisRelationshiptypeRelationshiptypedefinesa

setofassociationsorarelationshipsetamongagivensetofentitytypes.

27.WhatisdegreeofRelationshiptypeItisthenumberofentitytypeparticipating.

25.WhatisDDL（DataDefinitionLanguage）AdatabaseschemaisspecifiesbyasetofdefinitionsexpressedbyaspeciallanguagecalledDDL.

26.WhatisVDL（ViewDefinitionLanguage）Itspecifies

userviewsandtheirmappingstotheconceptualschema.

27.WhatisSDL（StorageDefinitionLanguage）ThisIanguageistospecifytheinternalschema.ThisIanguagemayspecifythemappingbetweentwoschemas.

28.WhatisDataStorageDefinitionLanguageThestoragestructuresandaccessmethodsusedbydatabasesystemarespecifiedbyasetofdefinitioninaspecialtypeofDDLcalled

datastorage-definitionIanguage.

29.WhatisDML（DataManipulationLanguage）ThisIanguagethatenableusertoaccessormanipulatedataasorganisedbyappropriatedatamodel.

ProceduralDMLorLowlevel:

DMLrequiresausertospecifywhatdataareneededandhowtogetthosedata.Non-ProceduralDMLorHighlevel:

DMLrequiresausertospecifywhatdataareneededwithoutspecifyinghowtogetthosedata.

31.WhatisDMLCompilerIttranslatesDMLstatementsin

aqueryIanguageintolow-levelinstructionthatthequery

evaluationenginecanunderstand.

32.WhatisQueryevaluationengineItexecuteslow-level

instructiongeneratedbycompiler.

33.WhatisDDLInterpreterItinterpretsDDLstatements

andrecordthemintablescontainingmetadata.

34.WhatisRecord-at-a-timeTheLowlevelorProcedural

DMLcanspecifyandretrieveeachrecordfromasetofrecords.

ThisretrieveofarecordissaidtobeRecord-at-a-time.

35.WhatisSet-at-a-timeorSet-orientedTheHighlevelor

Non-proceduralDMLcanspecifyandretrievemanyrecordsinasingleDMLstatement.Thisretrieveofarecordissaidtobe

Set-at-a-timeorSet-oriented.

36.WhatisRelationalAlgebraItisproceduralqueryIanguage.Itconsistsofasetofoperationsthattakeoneortworelationsasinputandproduceanewrelation.

37.WhatisRelationalCalculusItisanappliedpredicatecalculusspecificallytailoredforrelationaldatabasesproposed

byE.F.Codd.E.g.ofIanguagesbasedonitareDSLALPHA,QUEL.

38.HowdoesTuple-orientedrelationalcalculusdiffer

fromdomain-orientedrelationalcalculusThetuple-oriented

calculususesatuplevariablesi.e.,variablewhoseonlypermittedvaluesaretuplesofthatrelation.E.g.QUEL

Thedomain-orientedcalculushasdomainvariablesi.e.,variablesthatrangeovertheunderlyingdomainsinsteadofoverrelation.E.g.ILL,DEDUCE.

39.WhatisnormalizationItisaprocessofanalysingthe

givenrelationschemasbasedontheirFunctional

Dependencies（FDs）andprimarykeytoachievetheproperties

MinimizingredundancyMinimizinginsertion,deletionandupdateanomalies.

40.WhatisFunctionalDependencyAFunctional

dependencyisdenotedbyXYbetweentwosetsofattributes

XandYthataresubsetsofRspecifiesaconstraintonthe

possibletuplethatcanformarelationstaterofR.Theconstraintisforanytwotuplest1andt2inrift1[X]=t2[X]thentheyhavet1[Y]=t2[Y].ThismeansthevalueofXcomponentofatupleuniquelydeterminesthevalueof

componentY.

41.WhenisafunctionaldependencyFsaidtobeminimal

EverydependencyinFhasasingleattributeforitsrighthandside.WecannotreplaceanydependencyXAinFwithadependencyYAwhereYisapropersubsetofXandstillhaveasetofdependencythatisequivalenttoF.WecannotremoveanydependencyfromFandstillhavesetofdependencythatisequivalenttoF.

42.WhatisMultivalueddependencyMultivalueddependencydenotedbyXYspecifiedonrelationschemaR,

whereXandYarebothsubsetsofR,specifiesthefollowingconstraintonan