数据库管理系统外文翻译.docx
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数据库管理系统外文翻译.docx
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数据库管理系统外文翻译
数据库管理系统外文翻译
(文档含中英文对照即英文原文和中文翻译)
DatabaseManagementSystem
Source:
DatabaseandNetworkJournalAuthor:
DavidAnderson
Youknowthatadataisacollectionoflogicallyrelateddataelementsthatmaybestructuredinvariouswaystomeetthemultipleprocessingandretrievalneedsoforganizationsandindividuals.There’snothingnewaboutdatabase-earlyoneswerechiseledinstone,pennedonscrolls,andwrittenonindexcards.Butnowdatabasearecommonlyrecordedonmagneticallymedia,andcomputerprogramsarerequiredtoperformthenecessarystorageandretrievaloperations.
Thesystemsoftwarepackagethathandlesthedifficulttasksassociatedwithcreated,accessing,andmaintainingdatabaserecordsisinaDBMSpackageestablishaninterfacebetweenthedatabaseitselfandtheusersofthedatabase.(Theseusersmaybeapplicationsprogrammers,managersandotherswithinformationneeds,andvariousOSprogrammers.)
ADBMScanorganize,process,andpresentselecteddataelementsfromthedatabase.Thiscapabilityenablesdecisionmakerstosearch.Probe,andquerydatacontentsinordertoextractanswerstononrecurringandunplannedquestionsthataren’tavailableinregularreports.Thesequestionsmightinitiallybevagueand/orpoorlydefined,butpeoplecan“browse”throughthedatabaseuntiltheyhavetheneededinformation.Inshort,theDBMSwill“manage”thestoreddataitemsandassembletheneededitemsfromthecommondatabaseinresponsetothequeriesofthosewhoaren’tprogrammers.Inafile-orientedsystem,usersneedingspecialinformationmaycommunicatetheirneedstoaprogrammers,who,whentimepermits,willinformation.TheavailabilityofaDBMS,however,offersusersamuchfasteralternativecommunicationspatch(seefigure).
Special,direct,andotherfileprocessingapproachesateusedtoorganizeandstructuredatainsinglefiles.ButaDBMSisabletointegratedataelementsfromseveralfilestoanswerspecificuserinquiriesfirinformation.ThismeansthattheDBMSisabletostructureandtietogetherthelogicallyrelateddatafromseverallargefiles.
Logicalstructures.Identifyingtheselogicalrelationshipsisajobofthedataadministrator.Adatadefinitionlanguageisusedforthispurpose.TheDBMSmaythen
Employoneofthefollowinglogicalstructuringtechniquesduringstorageaccess,andretrievaloperation:
liststructures,hierarchical(tree)structures,andnetworkstructures,relationalstructures.
1.Liststructures.Inthislogicalapproach,recordsarelinkedtogetherbytheuseofpointers.Apointerisadataiteminonerecordthatidentifiesthestoragelocationofanotherlogicallyrelatedrecord.Recordsinacustomermasterfile,forexample,willcontainthenameandaddressofeachcustomer,andanaccountnumberidentifieseachrecordinthisfile.Duringanaccountingperiod,acustomermaymaintainaninvoicefiletoreflectthesetransactions.Aliststructurecouldbeusedinthissituationtoshowtheunpaidinvoicesatanygiventime.Eachinthecustomerfilewouldpointtotherecordlocationofthefirstinvoiceforthatcustomerintheinvoicefile.Thisinvoicerecord,inturnwouldbelinkedtolaterinvoiceforthecustomer.Thelastinvoiceinthechainwouldbeidentifiedbytheuseofaspecialcharacterasapointer.
2.Hierarchicalstructures.Inthislogicalapproach,dataunitsarestructuredinmultiplelevelsthatgraphicallyresemblean“upsidedown”treewiththerootatthetopandthebranchesformedbelow,there’sasuperior-subordinaterelationshipinahierarchicalstructure.Belowthesingle-rootdatacomponentaresubordinateelements(orone)hasonlyasingleowner.Thus,asweseeinfigure,acustomerownsaninvoice,andtheinvoicehassubordinateitems.Thebranchesinatreestructurearenotconnected.
3.Networkstructures.Unlikethetreeapproach,whichdosenotpermittheconnectionofbranches,thenetworkstructurepermitstheconnectionofthenodesinamultidirectionalmanner.Thus,eachnodemayhaveseveralownersandmay,inturn,ownanynumberofotherdataunits.Data,managementsoftwarepermitstheextractionoftheneededinformationfromsuchastructurebybeginningwithanyrecordinafile.
4.Relationalstructures.Arelationalstructureismadeupofmanytables.Thedataarestoredintheformof“relations”inthesetables.Forexample,relationtablescouldbeestablishedtolinkacollegecoursewiththeinstructorofthecourse,andwiththelocationoftheinordertofindthenameoftheinstructorandthelocationoftheEnglishclass,thecourse/instructorrelationissearchedtogetthename,andthecourse/locationrelationissearchedtogettheclasslocation.Manyotherrelationsareofcourse,possible.Thisisarelativelynewdatabasestructuringapproachthat’sexpectedtobewidelyimplementedinthefuture.
5.Physicalstructure.Peoplevisualizeorstructuredatainlogicalwaysforthere
Ownpurposes.Thus,recordsR1andR2mayalwaysbelogicallylinkedandprocessedinsequenceinoneparticularapplication.However,inacomputersystemit’squitepossiblethattheserecordsthatarelogicallycontiguousinoneapplicationarenotphysicallystoredtogether.Rather,thephysicalstructureoftheI/Oandstoragedevicestechniquesused,butalsoonthedifferentlogicalrelationshipsthatusersmayassigntothedatafoundonR1andR2.Forexample,R1andR2mayberecordsofcreditcustomerswhohaveshipmentssendtothesameblockinthesamecityeverytwoweeks.Fromtheshippingdepartmentmanager’sperspective,then,R1andR2aresequentialentriesonageographicallyorganizedshippingreport.Butmaybeidentified,andtheiraccountsmaybeprocessed,accordingtotheiraccountnumberswhicharewidelyseparated.Inshort,thenthephysicallocationofthestoredrecordsinmanycomputer-basedinformationsystemsisinvisibletousers.
Duringthepastfiveyears,MicrosofthaspromotedDataAccessObjects(DAO),andthenRemoteDataObjects(RDO),andnowActiveXDataObjects(ADO)astheprimarydataaccesstechnologyforVisualBasicdevelopers.ItseemsthatMicrosofthasbeenpushingadifferentdataaccesstechnologywitheachsuccessiveversionofMicrosoftVisualStudio.Today,newversionsofADOareavailableonMicrosoft'sWebsiteandshipwithotherproductsandtechnologies,suchasMicrosoftWindows2000,MicrosoftWindowsNT4ServicePacks,MicrosoftInternetExplorerversions3andlater,MicrosoftSQLServer6.5ServicePack5andSQLServer7,MicrosoftOffice2000,andevenMicrosoftExpediaStreets&Trips2000.
OneofthegoalsofADOistosimplifydataaccess.ADOisbuiltuponsomefairlycomplextechnologies—OLEDBandODBC(opendatabaseconnectivity)—andisdesignedtoallowyoutoprogrammaticallyaccessandmodifydatastoredinawidevarietyofdatabases.Thisbroadreachisadeparturefrompreviousdataaccesstechnologies.Forthesakeofcomparison,let'stakeaquickglanceatADO'spredecessors:
DAOandRDO.
DataAccessObjects
DAOwasoriginallydesignedtointeractwithMicrosoftAccessdatabases.AlthoughyoucanuseDAOtoaccessSQLServerandOracledatabases,manydeveloperscomplainaboutDAO'sperformancewiththeselargedatabasesystems.OtherscomplainthatDAOdoesn'tpermitprogrammerstoaccesssomeofthericher,morepowerfulfeaturesofSQLServerandOracle,suchasoutputandreturnparametersonstoredprocedures.
OneofmycoworkerslikestosaythatusingDAOtoworkwithanOracledatabaseislikeperformingbrainsurgeryonyou…withoutanesthetics…whilewearingovenmitts.Extreme?
Yes—buthedoeshaveapoint.DAOistunedtoworkwithdesktopdatabases,notclient/serverdatabases.FrustratedbyDAO'sperformanceandaccesslimitations,developerswhowantedtoworkwithSQLServerandOracledatabasesgenerallysoughtotheroptions.
RemoteDataObjects
MicrosoftprovidedanotheroptioninRDO,whichoriginallyreleasedwithVisualBasic4EnterpriseEdition.RDO'sobjectmodelcloselyresemblesthehierarchyofstructuresintheODBCAPI.ProgrammersfoundthatRDOprovidedmuchfasteraccesstoclient/serverdatabasesystems,suchasSQLServerandOracle,thanDAOdid.AlthoughthosefamiliarwiththeODBCAPIquicklylearnedhowtoworkwiththeRDOobjectmodel,developerslackingexperiencewiththatAPI,suchasthosewhohadbeenusingDAO,foundtheRDOtechnologydifficulttouse.
Theobjectmodelitselfwasn'ttheproblemformostprogrammerslearningRDO:
thenuancesinheritedfromtheODBCAPIposedthegreatestobstacles.Suddenly,programmershadtoboneuponcursorsandbookmarks.Theyhadtolearnmanyoftheinsandoutsofspecificdatabasesystems.Doestheerrormessage"Theconnectionisbusywithresultsfromanotherhstmt"ringanybellsoutthere?
IfyoutrytodotheimpossibleonanODBCconnectiontoyourdatabase,RDOwon'tsaveyou.Instead,you'llgetthaterror.DAOhidtheproblemfromyoubyautomaticallycreatinganotherconnectiontoyourdatabasetoperformtheactionyourequested.
AnotherchallengethatRDOposedforprogrammersaccustomedtowritingDAOcodewasthatRDOlackedmanyofDAO'sfeatures,suchassorting,searching,andfiltering.OtherDAOfunctionalityunavailableintheRDOworldincludesdatadefinitionlanguage(DDL)interfacestoODBCAPIfunctionssuchasCreateTableandCreateField.
BestofBothWorlds:
ActiveXDataObjects
ProgrammersclamoredforadataaccesstechnologythatcombinedthesimplicityandrelativeeaseofuseofDAOwiththespeed,power,andcontrolofRDO.InitiallyintroducedaspartoftheMicrosoftInternetInformationServer3package,ADOwasintendedtobeallthingstoallpeople.Ofcourse,suchloftygoalsarerarelyfulfilled.
Whilet
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