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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,thatisspeciallyorganizedforrapidsearchandretrievalbyacomputer.Databasesarestructuredtofacilitatethestorage,retrieval,modification,anddeletionofdatainconjunctionwithvariousdataprocessingoperations.Databasescanbestoredonmagneticdiskortape,opticaldisk,orsomeothersecondarystoragedevice.Adatabaseconsistsofafileorasetoffiles.Theinformationinthesefilesmaybebrokendownintorecords,eachofwhichconsistsofoneormorefields.Fieldsarethebasicunitsofdatastorage,andeachfieldtypicallycontainsinformationpertainingtooneaspectorattributeoftheentitydescribedbythedatabase.Usingkeywordsandvarioussortingcommands,userscanrapidlysearch,rearrange,group,andselectthefieldsinmanyrecordstoretrieveorcreatereportsonparticularaggregateofdata.Complexdatarelationshipsandlinkagesmaybefoundinallbutthesimplestdatabases.Thesystemsoftwarepackagethathandlesthedifficulttasksassociatedwithcreating,accessing,andmaintainingdatabaserecordsiscalledadatabasemanagementsystemDBMS.TheprogramsinaDBMSpackageestablishaninterfacebetweenthedatabaseitselfandtheusersofthedatabase..Theseusersmaybeapplicationsprogrammers,managersandotherswithinformationneeds,andvariousOSprograms.ADBMScanorganize,process,andpresentselecteddataelementsformthedatabase.Thiscapabilityenablesdecisionmakerstosearch,probe,andquerydatabasecontentsinordertoextractanswerstononrecurringandunplannedquestionsthatarentavailableinregularreports.Thesequestionsmightinitiallybevagueand/orpoorlydefined,butpeoplecanbrowsethroughthedatabaseuntiltheyhavetheneededinformation.Inshort,theDBMSwillmanagethestoreddataitemsandassembletheneededitemsfromthecommondatabaseinresponsetothequeriesofthosewhoarentprogrammers.AdatabasemanagementsystemDBMSiscomposedofthreemajorparts1astoragesubsystemthatstoresandretrievesdatainfiles2amodelingandmanipulationsubsystemthatprovidesthemeanswithwhichtoorganizethedataandtoadd,delete,maintain,andupdatethedata3andaninterfacebetweentheDBMSanditsusers.SeveralmajortrendsareemergingthatenhancethevalueandusefulnessofdatabasemanagementsystemsManagerswhorequiremoreuptodatainformationtomakeeffectivedecisionCustomerswhodemandincreasinglysophisticatedinformationservicesandmorecurrentinformationaboutthestatusoftheirorders,invoices,andaccounts.Userswhofindthattheycandevelopcustomapplicationswithdatabasesystemsinafractionofthetimeittakestousetraditionalprogramminglanguages.Organizationsthatdiscoverinformationhasastrategicvaluetheyutilizetheirdatabasesystemstogainanedgeovertheircompetitors.TheDatabaseModelAdatamodeldescribesawaytostructureandmanipulatethedatainadatabase.Thestructuralpartofthemodelspecifieshowdatashouldberepresentedsuchastree,tables,andsoon.Themanipulativepartofthemodelspecifiestheoperationwithwhichtoadd,delete,display,maintain,print,search,select,sortandupdatethedata.HierarchicalModelThefirstdatabasemanagementsystemsusedahierarchicalmodelthatistheyarrangedrecordsintoatreestructure.Somerecordsarerootrecordsandallothershaveuniqueparentrecords.Thestructureofthetreeisdesignedtoreflecttheorderinwhichthedatawillbeusedthatis,therecordattherootofatreewillbeaccessedfirst,thenrecordsonelevelbelowtheroot,andsoon.Thehierarchicalmodelwasdevelopedbecausehierarchicalrelationshipsarecommonlyfoundinbusinessapplications.Asyouhaveknown,anorganizationcharoftendescribesahierarchicalrelationshiptopmanagementisatthehighestlevel,middlemanagementatlowerlevels,andoperationalemployeesatthelowestlevels.Notethatwithinastricthierarchy,eachlevelofmanagementmayhavemanyemployeesorlevelsofemployeesbeneathit,buteachemployeehasonlyonemanager.Hierarchicaldataarecharacterizedbythisonetomanyrelationshipamongdata.Inthehierarchicalapproach,eachrelationshipmustbeexplicitlydefinedwhenthedatabaseiscreated.Eachrecordinahierarchicaldatabasecancontainonlyonekeyfieldandonlyonerelationshipisallowedbetweenanytwofields.Thiscancreateaproblembecausedatadonotalwaysconformtosuchastricthierarchy.RelationalModelAmajorbreakthroughindatabaseresearchoccurredin1970whenE.F.Coddproposedafundamentallydifferentapproachtodatabasemanagementcalledrelationalmodel,whichusesatableasitsdatastructure.Therelationaldatabaseisthemostwidelyuseddatabasestructure.Dataisorganizedintorelatedtables.Eachtableismadeupofrowscalledandcolumnscalledfields.Eachrecordcontainsfieldsofdataaboutsomespecificitem.Forexample,inatablecontaininginformationonemployees,arecordwouldcontainfieldsofdatasuchasapersonslastname,firstname,andstreetaddress.StructuredquerylanguageSQLisaquerylanguageformanipulatingdatainarelationaldatabase.Itisnonproceduralordeclarative,inwhichtheuserneedonlyspecifyanEnglishlikedescriptionthatspecifiestheoperationandthedescribedrecordorcombinationofrecords.Aqueryoptimizertranslatesthedescriptionintoaproceduretoperformthedatabasemanipulation.NetworkModelThenetworkmodelcreatesrelationshipsamongdatathroughalinkedliststructureinwhichsubordinaterecordscanbelinkedtomorethanoneparentrecord.Thisapproachcombinesrecordswithlinks,whicharecalledpointers.Thepointersareaddressesthatindicatethelocationofarecord.Withthenetworkapproach,asubordinaterecordcanbelinkedtoakeyrecordandatthesametimeitselfbeakeyrecordlinkedtoothersetsofsubordinaterecords.Thenetworkmodehistoricallyhashadaperformanceadvantageoverotherdatabasemodels.Today,suchperformancecharacteristicsareonlyimportantinhighvolume,highspeedtransactionprocessingsuchasautomatictellermachinenetworksorairlinereservationsystem.Bothhierarchicalandnetworkdatabasesareapplicationspecific.Ifanewapplicationisdeveloped,maintainingtheconsistencyofdatabasesindifferentapplicationscanbeverydifficult.Forexample,supposeanewpensionapplicationisdeveloped.Thedataarethesame,butanewdatabasemustbecreated.ObjectModelThenewestapproachtodatabasemanagementusesanobjectmodel,inwhichrecordsarerepresentedbyentitiescalledobjectsthatcanbothstoredataandprovidemethodsorprocedurestoperformspecifictasks.Thequerylanguageusedfortheobjectmodelisthesameobjectorientedprogramminglanguageusedtodevelopthedatabaseapplication.Thiscancreateproblemsbecausethereisnosimple,uniformquerylanguagesuchasSQL.Theobjectmodelisrelativelynew,andonlyafewexamplesofobjectorienteddatabaseexist.Ithasattractedattentionbecausedeveloperswhochooseanobjectorientedprogramminglanguagewantadatabasebasedonanobjectorientedmodel.DistributedDatabaseSimilarly,adistributeddatabaseisoneinwhichdifferentpartsofthedatabaseresideonphysicallyseparatedcomputers.Onegoalofdistributeddatabasesistheaccessofinformationwithoutregardtowherethedatamightbestored.Keepinginmindthatoncetheusersandtheirdataareseparated,thecommunicationandnetworkingconceptscomeintoplay.Distributeddatabasesrequiresoftwarethatresidespartiallyinthelargercomputer.Thissoftwarebridgesthegapbetweenpersonalandlargecomputersandresolvestheproblemsofincompatibledataformats.Ideally,itwouldmakethemainframedatabasesappeartobelargelibrariesofinformation,withmostoftheprocessingaccomplishedonthepersonalcomputer.Adrawbacktosomedistributedsystemsisthattheyareoftenbasedonwhatiscalledamainframeentiremodel,inwhichthelargerhostcomputerisseenasthemasterandtheterminalorpersonalcomputerisseenasaslave.Therearesomeadvantagestothisapproach.Withdatabasesundercentralizedcontrol,manyoftheproblemsofdataintegritythatwementionedearlieraresolved.Buttodayspersonalcomputers,departmentalcomputers,anddistributedprocessingrequirecomputersandtheirapplicationstocommunicatewitheachotheronamoreequalorpeertopeerbasis.Inadatabase,theclient/servermodelprovidestheframeworkfordistributingdatabases.Onewaytotakeadvantageofmanyconnectedcomputersrunningdatabaseapplicationsistodistributetheapplicationintocooperatingpartsthatareindependentofoneanther.Aclientisanenduserorcomputerprogramthatrequestsresourcesacrossanetwork.Aserverisacomputerrunningsoftwarethatfulfillsthoserequestsacrossanetwork.Whentheresourcesaredatainadatabase,theclient/servermodelprovidestheframeworkfordistributingdatabase.Afileserveissoftwarethatprovidesaccesstofilesacrossanetwork.Adedicatedfileserverisasinglecomputerdedicatedtobeingafileserver.Thisisuseful,forexample,ifthefilesarelargeandrequirefastaccess.Insuchcases,aminicomputerormainframewouldbeusedasafileserver.Adistributedfileserverspreadsthefilesaroundonindividualcomputersinsteadofplacingthemononededicatedcomputer.Advantagesofthelatterserverincludetheabilitytostoreandretrievefilesonothercomputersandtheeliminationofduplicatefilesoneachcomputer.Amajordisadvantage,however,isthatindividualread/writerequestsarebeingmovedacrossthenetworkandproblemscanarisewhenupdatingfiles.Supposeauserrequestsarecordfromafileandchangesitwhileanotheruserrequeststhesamerecordandchangesittoo.Thesolutiontothisproblemscalledrecordlocking,whichmeansthatthefirstrequestmakesothersrequestswaituntilthefirstrequestissatisfied.Otherusersmaybeabletoreadtherecord,buttheywillnotbeabletochangeit.Adatabaseserverissoftwarethatservicesrequeststoadatabaseacrossanetwork.Forexample,supposeausertypesinaqueryfordataonhisorherpersonalcomputer.Iftheapplicationisdesignedwiththeclient/servermodelinmind,thequerylanguagepartonthepersonalcomputersimplesendsthequeryacrossthenetworktothedatabaseserverandrequeststobenotifiedwhenthedataarefound.Examplesofdistributeddatabasesystemscanbefoundintheengineeringworld.SunsNetworkFilingSystemNFS,forexample,isusedincomputeraidedengineeringapplicationstodistributedataamongtheharddisksinanetworkofSunworkstation.Distributingdatabasesisanevolutionarystepbecauseitislogicalthatdatashouldexistatthelocationwheretheyarebeingused.Departmentalcomputerswithinalargecorporation,forexample,shouldhavedataresidelocally,yetthosedatashouldbeaccessiblebyauthorizedcorporatemanagementwhentheywanttoconsolidatedepartmentaldata.DBMSsoftwarewillprotectthesecurityandintegrityofthedatabase,andthedistributeddatabasewillappeartoitsusersasnodifferentfromthenondistributeddatabase.Inthisinformationage,thedataserverhasbecometheheartofacompany.Thisonepieceofsoftwarecontrolstherhythmofmostorganizationsandisusedtopumpinformationlifebloodthroughthearteriesofthenetwork.Becauseofthecriticalnatureofthisapplication,thedataserverisalsotheoneofthemostpopulartargetsforhackers.Ifahackerownsthis

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