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外文翻译--数据库管理系统介绍.doc

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外文翻译--数据库管理系统介绍.doc

原文DATABASEMANAGEMENTSYSTEMSADATABASESOMETIMESSPELLEDDATABASEISALSOCALLEDANELECTRONICDATABASE,REFERRINGTOANYCOLLECTIONOFDATA,ORINFORMATION,THATISSPECIALLYORGANIZEDFORRAPIDSEARCHANDRETRIEVALBYACOMPUTERDATABASESARESTRUCTUREDTOFACILITATETHESTORAGE,RETRIEVAL,MODIFICATION,ANDDELETIONOFDATAINCONJUNCTIONWITHVARIOUSDATAPROCESSINGOPERATIONSDATABASESCANBESTOREDONMAGNETICDISKORTAPE,OPTICALDISK,ORSOMEOTHERSECONDARYSTORAGEDEVICEADATABASECONSISTSOFAFILEORASETOFFILESTHEINFORMATIONINTHESEFILESMAYBEBROKENDOWNINTORECORDS,EACHOFWHICHCONSISTSOFONEORMOREFIELDSFIELDSARETHEBASICUNITSOFDATASTORAGE,ANDEACHFIELDTYPICALLYCONTAINSINFORMATIONPERTAININGTOONEASPECTORATTRIBUTEOFTHEENTITYDESCRIBEDBYTHEDATABASEUSINGKEYWORDSANDVARIOUSSORTINGCOMMANDS,USERSCANRAPIDLYSEARCH,REARRANGE,GROUP,ANDSELECTTHEFIELDSINMANYRECORDSTORETRIEVEORCREATEREPORTSONPARTICULARAGGREGATEOFDATACOMPLEXDATARELATIONSHIPSANDLINKAGESMAYBEFOUNDINALLBUTTHESIMPLESTDATABASESTHESYSTEMSOFTWAREPACKAGETHATHANDLESTHEDIFFICULTTASKSASSOCIATEDWITHCREATING,ACCESSING,ANDMAINTAININGDATABASERECORDSISCALLEDADATABASEMANAGEMENTSYSTEMDBMSTHEPROGRAMSINADBMSPACKAGEESTABLISHANINTERFACEBETWEENTHEDATABASEITSELFANDTHEUSERSOFTHEDATABASETHESEUSERSMAYBEAPPLICATIONSPROGRAMMERS,MANAGERSANDOTHERSWITHINFORMATIONNEEDS,ANDVARIOUSOSPROGRAMSADBMSCANORGANIZE,PROCESS,ANDPRESENTSELECTEDDATAELEMENTSFORMTHEDATABASETHISCAPABILITYENABLESDECISIONMAKERSTOSEARCH,PROBE,ANDQUERYDATABASECONTENTSINORDERTOEXTRACTANSWERSTONONRECURRINGANDUNPLANNEDQUESTIONSTHATAREN’TAVAILABLEINREGULARREPORTSTHESEQUESTIONSMIGHTINITIALLYBEVAGUEAND/ORPOORLYDEFINED,BUTPEOPLECAN“BROWSE”THROUGHTHEDATABASEUNTILTHEYHAVETHENEEDEDINFORMATIONINSHORT,THEDBMSWILL“MANAGE”THESTOREDDATAITEMSANDASSEMBLETHENEEDEDITEMSFROMTHECOMMONDATABASEINRESPONSETOTHEQUERIESOFTHOSEWHOAREN’TPROGRAMMERSADATABASEMANAGEMENTSYSTEMDBMSISCOMPOSEDOFTHREEMAJORPARTS1ASTORAGESUBSYSTEMTHATSTORESANDRETRIEVESDATAINFILES;2AMODELINGANDMANIPULATIONSUBSYSTEMTHATPROVIDESTHEMEANSWITHWHICHTOORGANIZETHEDATAANDTOADD,DELETE,MAINTAIN,ANDUPDATETHEDATA;3ANDANINTERFACEBETWEENTHEDBMSANDITSUSERSSEVERALMAJORTRENDSAREEMERGINGTHATENHANCETHEVALUEANDUSEFULNESSOFDATABASEMANAGEMENTSYSTEMS;MANAGERSWHOREQUIREMOREUPTODATAINFORMATIONTOMAKEEFFECTIVEDECISIONCUSTOMERSWHODEMANDINCREASINGLYSOPHISTICATEDINFORMATIONSERVICESANDMORECURRENTINFORMATIONABOUTTHESTATUSOFTHEIRORDERS,INVOICES,ANDACCOUNTSUSERSWHOFINDTHATTHEYCANDEVELOPCUSTOMAPPLICATIONSWITHDATABASESYSTEMSINAFRACTIONOFTHETIMEITTAKESTOUSETRADITIONALPROGRAMMINGLANGUAGESORGANIZATIONSTHATDISCOVERINFORMATIONHASASTRATEGICVALUE;THEYUTILIZETHEIRDATABASESYSTEMSTOGAINANEDGEOVERTHEIRCOMPETITORSTHEDATABASEMODELADATAMODELDESCRIBESAWAYTOSTRUCTUREANDMANIPULATETHEDATAINADATABASETHESTRUCTURALPARTOFTHEMODELSPECIFIESHOWDATASHOULDBEREPRESENTEDSUCHASTREE,TABLES,ANDSOONTHEMANIPULATIVEPARTOFTHEMODELSPECIFIESTHEOPERATIONWITHWHICHTOADD,DELETE,DISPLAY,MAINTAIN,PRINT,SEARCH,SELECT,SORTANDUPDATETHEDATAHIERARCHICALMODELTHEFIRSTDATABASEMANAGEMENTSYSTEMSUSEDAHIERARCHICALMODELTHATISTHEYARRANGEDRECORDSINTOATREESTRUCTURESOMERECORDSAREROOTRECORDSANDALLOTHERSHAVEUNIQUEPARENTRECORDSTHESTRUCTUREOFTHETREEISDESIGNEDTOREFLECTTHEORDERINWHICHTHEDATAWILLBEUSEDTHATIS,THERECORDATTHEROOTOFATREEWILLBEACCESSEDFIRST,THENRECORDSONELEVELBELOWTHEROOT,ANDSOONTHEHIERARCHICALMODELWASDEVELOPEDBECAUSEHIERARCHICALRELATIONSHIPSARECOMMONLYFOUNDINBUSINESSAPPLICATIONSASYOUHAVEKNOWN,ANORGANIZATIONCHAROFTENDESCRIBESAHIERARCHICALRELATIONSHIPTOPMANAGEMENTISATTHEHIGHESTLEVEL,MIDDLEMANAGEMENTATLOWERLEVELS,ANDOPERATIONALEMPLOYEESATTHELOWESTLEVELSNOTETHATWITHINASTRICTHIERARCHY,EACHLEVELOFMANAGEMENTMAYHAVEMANYEMPLOYEESORLEVELSOFEMPLOYEESBENEATHIT,BUTEACHEMPLOYEEHASONLYONEMANAGERHIERARCHICALDATAARECHARACTERIZEDBYTHISONETOMANYRELATIONSHIPAMONGDATAINTHEHIERARCHICALAPPROACH,EACHRELATIONSHIPMUSTBEEXPLICITLYDEFINEDWHENTHEDATABASEISCREATEDEACHRECORDINAHIERARCHICALDATABASECANCONTAINONLYONEKEYFIELDANDONLYONERELATIONSHIPISALLOWEDBETWEENANYTWOFIELDSTHISCANCREATEAPROBLEMBECAUSEDATADONOTALWAYSCONFORMTOSUCHASTRICTHIERARCHYRELATIONALMODELAMAJORBREAKTHROUGHINDATABASERESEARCHOCCURREDIN1970WHENEFCODDPROPOSEDAFUNDAMENTALLYDIFFERENTAPPROACHTODATABASEMANAGEMENTCALLEDRELATIONALMODEL,WHICHUSESATABLEASITSDATASTRUCTURETHERELATIONALDATABASEISTHEMOSTWIDELYUSEDDATABASESTRUCTUREDATAISORGANIZEDINTORELATEDTABLESEACHTABLEISMADEUPOFROWSCALLEDANDCOLUMNSCALLEDFIELDSEACHRECORDCONTAINSFIELDSOFDATAABOUTSOMESPECIFICITEMFOREXAMPLE,INATABLECONTAININGINFORMATIONONEMPLOYEES,ARECORDWOULDCONTAINFIELDSOFDATASUCHASAPERSON’SLASTNAME,FIRSTNAME,ANDSTREETADDRESSSTRUCTUREDQUERYLANGUAGESQLISAQUERYLANGUAGEFORMANIPULATINGDATAINARELATIONALDATABASEITISNONPROCEDURALORDECLARATIVE,INWHICHTHEUSERNEEDONLYSPECIFYANENGLISHLIKEDESCRIPTIONTHATSPECIFIESTHEOPERATIONANDTHEDESCRIBEDRECORDORCOMBINATIONOFRECORDSAQUERYOPTIMIZERTRANSLATESTHEDESCRIPTIONINTOAPROCEDURETOPERFORMTHEDATABASEMANIPULATIONNETWORKMODELTHENETWORKMODELCREATESRELATIONSHIPSAMONGDATATHROUGHALINKEDLISTSTRUCTUREINWHICHSUBORDINATERECORDSCANBELINKEDTOMORETHANONEPARENTRECORDTHISAPPROACHCOMBINESRECORDSWITHLINKS,WHICHARECALLEDPOINTERSTHEPOINTERSAREADDRESSESTHATINDICATETHELOCATIONOFARECORDWITHTHENETWORKAPPROACH,ASUBORDINATERECORDCANBELINKEDTOAKEYRECORDANDATTHESAMETIMEITSELFBEAKEYRECORDLINKEDTOOTHERSETSOFSUBORDINATERECORDSTHENETWORKMODEHISTORICALLYHASHADAPERFORMANCEADVANTAGEOVEROTHERDATABASEMODELSTODAY,SUCHPERFORMANCECHARACTERISTICSAREONLYIMPORTANTINHIGHVOLUME,HIGHSPEEDTRANSACTIONPROCESSINGSUCHASAUTOMATICTELLERMACHINENETWORKSORAIRLINERESERVATIONSYSTEMBOTHHIERARCHICALANDNETWORKDATABASESAREAPPLICATIONSPECIFICIFANEWAPPLICATIONISDEVELOPED,MAINTAININGTHECONSISTENCYOFDATABASESINDIFFERENTAPPLICATIONSCANBEVERYDIFFICULTFOREXAMPLE,SUPPOSEANEWPENSIONAPPLICATIONISDEVELOPEDTHEDATAARETHESAME,BUTANEWDATABASEMUSTBECREATEDOBJECTMODELTHENEWESTAPPROACHTODATABASEMANAGEMENTUSESANOBJECTMODEL,INWHICHRECORDSAREREPRESENTEDBYENTITIESCALLEDOBJECTSTHATCANBOTHSTOREDATAANDPROVIDEMETHODSORPROCEDURESTOPERFORMSPECIFICTASKSTHEQUERYLANGUAGEUSEDFORTHEOBJECTMODELISTHESAMEOBJECTORIENTEDPROGRAMMINGLANGUAGEUSEDTODEVELOPTHEDATABASEAPPLICATIONTHISCANCREATEPROBLEMSBECAUSETHEREISNOSIMPLE,UNIFORMQUERYLANGUAGESUCHASSQLTHEOBJECTMODELISRELATIVELYNEW,ANDONLYAFEWEXAMPLESOFOBJECTORIENTEDDATABASEEXISTITHASATTRACTEDATTENTIONBECAUSEDEVELOPERSWHOCHOOSEANOBJECTORIENTEDPROGRAMMINGLANGUAGEWANTADATABASEBASEDONANOBJECTORIENTEDMODELDISTRIBUTEDDATABASESIMILARLY,ADISTRIBUTEDDATABASEISONEINWHICHDIFFERENTPARTSOFTHEDATABASERESIDEONPHYSICALLYSEPARATEDCOMPUTERSONEGOALOFDISTRIBUTEDDATABASESISTHEACCESSOFINFORMATIONWITHOUTREGARDTOWHERETHEDATAMIGHTBESTOREDKEEPINGINMINDTHATONCETHEUSERSANDTHEIRDATAARESEPARATED,THECOMMUNICATIONANDNETWORKINGCONCEPTSCOMEINTOPLAYDISTRIBUTEDDATABASESREQUIRESOFTWARETHATRESIDESPARTIALLYINTHELARGERCOMPUTERTHISSOFTWAREBRIDGESTHEGAPBETWEENPERSONALANDLARGECOMPUTERSANDRESOLVESTHEPROBLEMSOFINCOMPATIBLEDATAFORMATSIDEALLY,ITWOULDMAKETHEMAINFRAMEDATABASESAPPEARTOBELARGELIBRARIESOFINFORMATION,WITHMOSTOFTHEPROCESSINGACCOMPLISHEDONTHEPERSONALCOMPUTERADRAWBACKTOSOMEDISTRIBUTEDSYSTEMSISTHATTHEYAREOFTENBASEDONWHATISCALLEDAMAINFRAMEENTIREMODEL,INWHICHTHELARGERHOSTCOMPUTERISSEENASTHEMASTERANDTHETERMINALORPERSONALCOMPUTERISSEENASASLAVETHEREARESOMEADVANTAGESTOTHISAPPROACHWITHDATABASESUNDERCENTRALIZEDCONTROL,MANYOFTHEPROBLEMSOFDATAINTEGRITYTHATWEMENTIONEDEARLIERARESOLVEDBUTTODAY’SPERSONALCOMPUTERS,DEPARTMENTALCOMPUTERS,ANDDISTRIBUTEDPROCESSINGREQUIRECOMPUTERSANDTHEIRAPPLICATIONSTOCOMMUNICATEWITHEACHOTHERONAMOREEQUALORPEERTOPEERBASISINADATABASE,THECLIENT/SERVERMODELPROVIDESTHEFRAMEWORKFORDISTRIBUTINGDATABASESONEWAYTOTAKEADVANTAGEOFMANYCONNECTEDCOMPUTERSRUNNINGDATABASEAPPLICATIONSISTODISTRIBUTETHEAPPLICATIONINTOCOOPERATINGPARTSTHATAREINDEPENDENTOFONEANTHERACLIENTISANENDUSERORCOMPUTERPROGRAMTHATREQUESTSRESOURCESACROSSANETWORKASERVERISACOMPUTERRUNNINGSOFTWARETHATFULFILLSTHOSEREQUESTSACROSSANETWORKWHENTHERESOURCESAREDATAINADATABASE,THECLIENT/SERVERMODELPROVIDESTHEFRAMEWORKFORDISTRIBUTINGDATABASEAFILESERVEISSOFTWARETHATPROVIDESACCESSTOFILESACROSSANETWORKADEDICATEDFILESERVERISASINGLECOMPUTERDEDICATEDTOBEINGAFILESERVERTHISISUSEFUL,FOREXAMPLE,IFTHEFILESARELARGEANDREQUIREFASTACCESSINSUCHCASES,AMINICOMPUTERORMAINFRAMEWOULDBEUSEDASAFILESERVERADISTRIBUTEDFILESERVERSPREADSTHEFILESAROUNDONINDIVIDUALCOMPUTERSINSTEADOFPLACINGTHEMONONEDEDICATEDCOMPUTERADVANTAGESOFTHELATTERSERVERINCLUDETHEABILITYTOSTOREANDRETRIEVEFILESONOTHERCOMPUTERSANDTHEELIMINATIONOFDUPLICATEFILESONEACHCOMPUTERAMAJORDISADVANTAGE,HOWEVER,ISTHATINDIVIDUALREAD/WRITEREQUESTSAREBEINGMOVEDACROSSTHENETWORKANDPROBLEMSCANARISEWHENUPDATINGFILESSUPPOSEAUSERREQUESTSARECORDFROMAFILEANDCHANGESITWHILEANOTHERUSERREQUESTSTHESAMERECORDANDCHANGESITTOOTHESOLUTIONTOTHISPROBLEMSCALLEDRECORDLOCKING,WHICHMEANSTHATTHEFIRSTREQUESTMAKESOTHERSREQUESTSWAITUNTILTHEFIRSTREQUESTISSATISFIEDOTHERUSERSMAYBEABLETOREADTHERECORD,BUTTHEYWILLNOTBEABLETOCHANGEITADATABASESERVERISSOFTWARETHATSERVICESREQUESTSTOADATABASEACROSSANETWORKFOREXAMPLE,SUPPOSEAUSERTYPESINAQUERYFORDATAONHISORHERPERSONALCOMPUTERIFTHEAPPLICATIONISDESIGNEDWITHTHECLIENT/SERVERMODELINMIND,THEQUERYLANGUAGEPARTONTHEPERSONALCOMPUTERSIMPLESENDSTHEQUERYACROSSTHENETWORKTOTHEDATABASESERVERANDREQUESTSTOBENOTIFIEDWHENTHEDATAAREFOUNDEXAMPLESOFDISTRIBUTEDDATABASESYSTEMSCANBEFOUNDINTHEENGINEERINGWORLDSUN’SNETWORKFILINGSYSTEMNFS,FOREXAMPLE,ISUSEDINCOMPUTERAIDEDENGINEERINGAPPLICATIONSTODISTRIBUTEDATAAMONGTHEHARDDISKSINANETWORKOFSUNWORKSTATIONDISTRIBUTINGDATABASESISANEVOLUTIONARYSTEPBECAUSEITISLOGICALTHATDATASHOULDEXISTATTHELOCATIONWHERETHEYAREBEINGUSEDDEPARTMENTALCOMPUTERSWITHINALARGECORPORATION,FOREXAMPLE,SHOULDHAVEDATARESIDELOCALLY,YETTHOSEDATASHOULDBEACCESSIBLEBYAUTHORIZEDCORPORATEMANAGEMENTWHENTHEYWANTTOCONSOLIDATEDEPARTMENTALDATADBMSSOFTWAREWILLPROTECTTHESECURITYANDINTEGRITYOFTHEDATABASE,ANDTHEDISTRIBUTEDDATABASEWILLAPPEARTOITSUSERSASNODIFFERENTFROMTHENONDISTRIBUTEDDATABASEINTHISINFORMATIONAGE,THEDATASERVERHASBECOMETHEHEARTOFACOMPANYTHISONEPIECEOFSOFTWARECONTROLSTHERHYTHMOFMOSTORGANIZATIONSANDISUSEDTOPUMPINFORMATIONLIFEBLOODTHROUGHTHEARTERIESOFTHENETWORKBECAUSEOFTHECRITICALNATUREOFTHISAPPLICATION,THEDATASERVERISALSOTHEONEOFTHEMOSTPOPULARTARGETSFORHACK

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