外文翻译数据库管理系统.docx

外文翻译数据库管理系统.docx

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外文翻译数据库管理系统

DatabaseManagementSystem

Source：

DatabaseandNetworkJournal

Author：

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.

WhiletheinitialreleaseofADOlackedmany