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外文资料原文DatabaseSystems1.IntroductiontoDatabaseSystemToday,morethanatanyprevioustime,thesuccessofanorganizationdependsonitsabilitytoacquireaccurateandtimelydataaboutitsoperation，tomanagethisdataeffectively，andtouseittoanalyzeandguideitsactivities.Phrasessuchastheinformationsuperhighwayhavebecomeubiquitous，andinformationprocessingisarapidlygrowingmultibilliondollarindustry。Theamountofinformationavailabletousisliterallyexploding,andthevalueofdataasanorganizationalassetisbeingwidelyrecognized。Thisparadoxdrivestheneedforincreasinglypowerfulandflexibledatamanagementsystems。Adatabaseisacollectionofdata,typicallydescribingtheactivitiesofoneormorerelatedorganizations.Forexample,auniversitydatabasemightcontaininformationaboutthefollowing.●Entitiessuchasstudents，faculty,courses，andclassrooms。●Relationshipsbetweenentities,suchasstudents'enrollmentincourses,facultyteachingcourses，andtheuseofroomsforcourses.Adatabasemanagementsystem,orDBMS,issoftwaredesignedtoassistinmaintainingandutilizinglargecollectionsofdata,andtheneedforsuchsystems，aswellastheiruse，isgrowingrapidly。ThealternativetousingaDBMSistouseadhocapproachesthatdonotcarryoverfromoneapplicationtoanother,forexample,tostorethedatainfilesandwriteapplication-specificcodetomanageit。Theareaofdatabasemanagementsystemsisamicrocosmofcomputerscienceingeneral.Theissuesaddressedandthetechniquesusedspanawidespectrum,includinglanguages，object-orientationandotherprogrammingparadigms,compilation，operatingsystems1concurrentprogramming,datastructures,algorithms,theory,parallelanddistributedsystems，userinterfaces,expertsystemsandartificialintelligence,statisticaltechniques,anddynamicprogramming.Databasemanagementcontinuestogainimportanceasmoreandmoredataisbroughton-line,andmadeevermoreaccessiblethroughcomputernetworking。Todaythefieldisbeingdrivenbyexcitingvisionssuchasmultimediadatabases,interactivevideo,digitallibraries,ahostofscientificprojectssuchasthehumangenomemappingeffortandNASA’sEarthObservationSystemproject，andthedesireofcompaniestoconsolidatetheirdecision-makingprocessesandminetheirdatarepositoriesforusefulinformationabouttheirbusiness.Commercially，databasemanagementsystemsrepresentoneofthelargestandmostvigorousmarketsegments.Thusthestudyofdatabasesystemscouldprovetoberichlyrewardinginmorewaysthanone.2.DatabaseconsistsAdatabaseconsistsofafileorasetoffiles。Theinformationinthesefilesmaybebrokendownintorecords,eachofwhichconsistsofoneormorefields。Fieldsarethebasicunitsofdatastorage，andeachfieldtypicallycontainsinformationpertainingtooneaspectorattributeoftheentitydescribedbythedatabase.Usingkeywordsandvarioussortingcommands，userscanrapidlysearch，rearrange，group，andselectthefieldsinmanyrecordstoretrieveorcreatereportsonparticularaggregatesofdata。Databaserecordsandfilesmustbeorganizedtoallowretrievaloftheinformation。Earlysystemswerearrangedsequentially（i。e。,alphabetically,numerically,orchronologically);thedevelopmentofdirect-accessstoragedevicesmadepossiblerandomaccesstodataviaindexes。Queriesarethemainwayusersretrievedatabaseinformation。Typically,theuserprovidesastringofcharacters,andthecomputer2searchesthedatabaseforacorrespondingsequenceandprovidesthesourcematerialsinwhichthosecharactersappear。Ausercanrequest，forexample,allrecordsinwhichthecontentofthefieldforaperson’slastnameisthewordSmith。...