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。...