数据库 外文翻译.docx

数据库 外文翻译.docx

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数据库外文翻译

AintroductiontoDatabaseManagementSystem

RaghuRamakrishnan

Adatabase（sometimesspelleddatabase）isalsocalledanelectronicdatabase,referringtoanycollectionofdata,orinformation,thatisspeciallyorganizedforrapidsearchandretrievalbyacomputer.Databasesarestructuredtofacilitatethestorage,retrieval,modification,anddeletionofdatainconjunctionwithvariousdata-processingoperations.Databasescanbestoredonmagneticdiskortape,opticaldisk,orsomeothersecondarystoragedevice.

Adatabaseconsistsofafileorasetoffiles.Theinformationinthesefilesmaybebrokendownintorecords,eachofwhichconsistsofoneormorefields.Fieldsarethebasicunitsofdatastorage,andeachfieldtypicallycontainsinformationpertainingtooneaspectorattributeoftheentitydescribedbythedatabase.Usingkeywordsandvarioussortingcommands,userscanrapidlysearch,rearrange,group,andselectthefieldsinmanyrecordstoretrieveorcreatereportsonparticularaggregateofdata.

Complexdatarelationshipsandlinkagesmaybefoundinallbutthesimplestdatabases.Thesystemsoftwarepackagethathandlesthedifficulttasksassociatedwithcreating,accessing,andmaintainingdatabaserecordsiscalledadatabasemanagementsystem（DBMS）.TheprogramsinaDBMSpackageestablishaninterfacebetweenthedatabaseitselfandtheusersofthedatabase..（Theseusersmaybeapplicationsprogrammers,managersandotherswithinformationneeds,andvariousOSprograms.）

ADBMScanorganize,process,andpresentselecteddataelementsformthedatabase.Thiscapabilityenablesdecisionmakerstosearch,probe,andquerydatabasecontentsinordertoextractanswerstononrecurringandunplannedquestionsthataren’tavailableinregularreports.Thesequestionsmightinitiallybevagueand/orpoorlydefined,butpeoplecan“browse”throughthedatabaseuntiltheyhavetheneededinformation.Inshort,theDBMSwill“manage”thestoreddataitemsandassembletheneededitemsfromthecommondatabaseinresponsetothequeriesofthosewhoaren’tprogrammers.

Adatabasemanagementsystem（DBMS）iscomposedofthreemajorparts:

（1）astoragesubsystemthatstoresandretrievesdatainfiles;

（2）amodelingandmanipulationsubsystemthatprovidesthemeanswithwhichtoorganizethedataandtoadd,delete,maintain,andupdatethedata;（3）andaninterfacebetweentheDBMSanditsusers.Severalmajortrendsareemergingthatenhancethevalueandusefulnessofdatabasemanagementsystems;

Managers:

whorequiremoreup-to-datainformationtomakeeffectivedecision

Customers:

whodemandincreasinglysophisticatedinformationservicesandmorecurrentinformationaboutthestatusoftheirorders,invoices,andaccounts.

Users:

whofindthattheycandevelopcustomapplicationswithdatabasesystemsinafractionofthetimeittakestousetraditionalprogramminglanguages.

Organizations:

thatdiscoverinformationhasastrategicvalue;theyutilizetheirdatabasesystemstogainanedgeovertheircompetitors.

TheDatabaseModel

Adatamodeldescribesawaytostructureandmanipulatethedatainadatabase.Thestructuralpartofthemodelspecifieshowdatashouldberepresented（suchastree,tables,andsoon）.Themanipulativepartofthemodelspecifiestheoperationwithwhichtoadd,delete,display,maintain,print,search,select,sortandupdatethedata.

HierarchicalModel

Thefirstdatabasemanagementsystemsusedahierarchicalmodel-thatis-theyarrangedrecordsintoatreestructure.Somerecordsarerootrecordsandallothershaveuniqueparentrecords.Thestructureofthetreeisdesignedtoreflecttheorderinwhichthedatawillbeusedthatis,therecordattherootofatreewillbeaccessedfirst,thenrecordsonelevelbelowtheroot,andsoon.

Thehierarchicalmodelwasdevelopedbecausehierarchicalrelationshipsarecommonlyfoundinbusinessapplications.Asyouhaveknown,anorganizationcharoftendescribesahierarchicalrelationship:

topmanagementisatthehighestlevel,middlemanagementatlowerlevels,andoperationalemployeesatthelowestlevels.Notethatwithinastricthierarchy,eachlevelofmanagementmayhavemanyemployeesorlevelsofemployeesbeneathit,buteachemployeehasonlyonemanager.Hierarchicaldataarecharacterizedbythisone-to-manyrelationshipamongdata.

Inthehierarchicalapproach,eachrelationshipmustbeexplicitlydefinedwhenthedatabaseiscreated.Eachrecordinahierarchicaldatabasecancontainonlyonekeyfieldandonlyonerelationshipisallowedbetweenanytwofields.Thiscancreateaproblembecausedatadonotalwaysconformtosuchastricthierarchy.

RelationalModel

Amajorbreakthroughindatabaseresearchoccurredin1970whenE.F.Coddproposedafundamentallydifferentapproachtodatabasemanagementcalledrelationalmodel,whichusesatableasitsdatastructure.

Therelationaldatabaseisthemostwidelyuseddatabasestructure.Dataisorganizedintorelatedtables.Eachtableismadeupofrowscalledandcolumnscalledfields.Eachrecordcontainsfieldsofdataaboutsomespecificitem.Forexample,inatablecontaininginformationonemployees,arecordwouldcontainfieldsofdatasuchasaperson’slastname,firstname,andstreetaddress.

Structuredquerylanguage（SQL）isaquerylanguageformanipulatingdatainarelationaldatabase.Itisnonproceduralordeclarative,inwhichtheuserneedonlyspecifyanEnglish-likedescriptionthatspecifiestheoperationandthedescribedrecordorcombinationofrecords.Aqueryoptimizertranslatesthedescriptionintoaproceduretoperformthedatabasemanipulation.

NetworkModel

Thenetworkmodelcreatesrelationshipsamongdatathroughalinked-liststructureinwhichsubordinaterecordscanbelinkedtomorethanoneparentrecord.Thisapproachcombinesrecordswithlinks,whicharecalledpointers.Thepointersareaddressesthatindicatethelocationofarecord.Withthenetworkapproach,asubordinaterecordcanbelinkedtoakeyrecordandatthesametimeitselfbeakeyrecordlinkedtoothersetsofsubordinaterecords.Thenetworkmodehistoricallyhashadaperformanceadvantageoverotherdatabasemodels.Today,suchperformancecharacteristicsareonlyimportantinhigh-volume,high-speedtransactionprocessingsuchasautomatictellermachinenetworksorairlinereservationsystem.

Bothhierarchicalandnetworkdatabasesareapplicationspecific.Ifanewapplicationisdeveloped,maintainingtheconsistencyofdatabasesindifferentapplicationscanbeverydifficult.Forexample,supposeanewpensionapplicationisdeveloped.Thedataarethesame,butanewdatabasemustbecreated.

ObjectModel

Thenewestapproachtodatabasemanagementusesanobjectmodel,inwhichrecordsarerepresentedbyentitiescalledobjectsthatcanbothstoredataandprovidemethodsorprocedurestoperformspecifictasks.

Thequerylanguageusedfortheobjectmodelisthesameobject-orientedprogramminglanguageusedtodevelopthedatabaseapplication.Thiscancreateproblemsbecausethereisnosimple,uniformquerylanguagesuchasSQL.Theobjectmodelisrelativelynew,andonlyafewexamplesofobject-orienteddatabaseexist.Ithasattractedattentionbecausedeveloperswhochooseanobject-orientedprogramminglanguagewantadatabasebasedonanobject-orientedmodel.

DistributedDatabase

Similarly,adistributeddatabaseisoneinwhichdifferentpartsofthedatabaseresideonphysicallyseparatedcomputers.Onegoalofdistributeddatabasesistheaccessofinformationwithoutregardtowherethedatamightbestored.Keepinginmindthatoncetheusersandtheirdataareseparated,thecommunicationandnetworkingconceptscomeintoplay.

Distributeddatabasesrequiresoftwarethatresidespartiallyinthelargercomputer.Thissoftwarebridgesthegapbetweenpersonalandlargecomputersandresolvestheproblemsofincompatibledataformats.Ideally,itwouldmakethemainframedatabasesappeartobelargelibrariesofinformation,withmostoftheprocessingaccomplishedonthepersonalcomputer.

Adrawbacktosomedistributedsystemsisthattheyareoftenbasedonwhatiscalledamainframe-entiremodel,inwhichthelargerhostcomputerisseenasthemasterandtheterminalorpersonalcomputerisseenasaslave.Therearesomeadvantagestothisapproach.Withdatabasesundercentralizedcontrol,manyoftheproblemsofdataintegritythatwementionedearlieraresolved.Buttoday’spersonalcomputers,departmentalcomputers,anddistributedprocessingrequirecomputersandtheirapplicationstocommunicatewitheachotheronamoreequalorpeer-to-peerbasis.Inadatabase,theclient/servermodelprovidestheframeworkfordistributingdatabases.

Onewaytotakeadvantageofmanyconnectedcomputersrunningdatabaseapplicationsistodistributetheapplicationintocooperatingpartsthatareindependentofoneanther.Aclientisanenduserorcomputerprogramthatrequestsresourcesacrossanetwork.Aserverisacomputerrunningsoftwarethatfulfillsthoserequestsacrossanetwork.Whentheresourcesaredatainadatabase,theclient/servermodelprovidestheframeworkfordistributingdatabase.

Afileserveissoftwarethatprovidesaccesstofilesacrossanetwork.Adedicatedfileserverisasinglecomputerdedicatedtobeingafileserver.Thisisuseful,forexample,ifthefilesarelargeandrequirefastaccess.Insuchcases,aminicomputerormainframewouldbeusedasafileserver.Adistributedfileserverspreadsthefilesaroundonindividualcomputersinsteadofplacingthemononededicatedcomputer.

Advantagesofthelatterserverincludetheabilitytostoreandretrievefilesonother