外文翻译-数据库管理系统—

大连交通大学2012届本科生毕业设计(论文)外文文献翻译1DatabaseManagementSystemSource：DatabaseandNetworkJournalAuthor：DavidAndersonYouknowthatadataisacollectionoflogicallyrelateddataelementsthatmaybestructuredinvariouswaystomeetthemultipleprocessingandretrievalneedsoforganizationsandindividuals.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.TheDBMSmay大连交通大学2012届本科生毕业设计(论文)外文文献翻译2thenEmployoneofthefollowinglogicalstructuringtechniquesduringstorageaccess,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’s大连交通大学2012届本科生毕业设计(论文)外文文献翻译3expectedtobewidelyimplementedinthefuture.5.Physicalstructure.PeoplevisualizeorstructuredatainlogicalwaysforthereOwnpurposes.Thus,recordsR1andR2mayalwaysbelogicallylinkedandprocessedinsequenceinoneparticularapplication.However,inacomputersystemit’squitepossiblethattheserecordsthatarelogicallycontiguousinoneapplicationarenotphysicallystoredtogether.Rather,thephysicalstructureoftheI/Oandstoragedevicestechniquesused,butalsoonthedifferentlogicalrelationshipsthatusersmayassigntothedatafoundonR1andR2.Forexample,R1andR2mayberecordsofcreditcustomerswhohaveshipmentssendtothesameblockinthesamecityeverytwoweeks.Fromtheshippingdepartmentmanager’sperspective,then,R1andR2aresequentialentriesonageographicallyorganizedshippingreport.Butmaybeidentified,andtheiraccountsmaybeprocessed,accordingtotheiraccountnu