计算机外文翻译外文文献英文文献数据库系统.docx
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计算机外文翻译外文文献英文文献数据库系统
外文资料原文
DatabaseSystems
1.IntroductiontoDatabaseSystem
Today,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,operatingsystemsconcurrentprogramming,datastructures,algorithms,theory,parallelanddistributedsystems,userinterfaces,expertsystemsandartificialintelligence,statisticaltechniques,anddynamicprogramming.
Databasemanagementcontinuestogainimportanceasmoreandmoredataisbroughton-line,andmadeevermoreaccessiblethroughcomputernetworking.Todaythefieldisbeingdrivenbyexcitingvisionssuchasmultimediadatabases,interactivevideo,digitallibraries,ahostofscientificprojectssuchasthehumangenomemappingeffortandNASA’sEarthObservationSystemproject,andthedesireofcompaniestoconsolidatetheirdecision-makingprocessesandminetheirdatarepositoriesforusefulinformationabouttheirbusiness.Commercially,databasemanagementsystemsrepresentoneofthelargestandmostvigorousmarketsegments.Thusthestudyofdatabasesystemscouldprovetoberichlyrewardinginmorewaysthanone.
2.Databaseconsists
Adatabaseconsistsofafileorasetoffiles.Theinformationinthesefilesmaybebrokendownintorecords,eachofwhichconsistsofoneormorefields.Fieldsarethebasicunitsofdatastorage,andeachfieldtypicallycontainsinformationpertainingtooneaspectorattributeoftheentitydescribedbythedatabase.Usingkeywordsandvarioussortingcommands,userscanrapidlysearch,rearrange,group,andselectthefieldsinmanyrecordstoretrieveorcreatereportsonparticularaggregatesofdata.
Databaserecordsandfilesmustbeorganizedtoallowretrievaloftheinformation.Earlysystemswerearrangedsequentially(i.e.,alphabetically,numerically,orchronologically);thedevelopmentofdirect-accessstoragedevicesmadepossiblerandomaccesstodataviaindexes.Queriesarethemainwayusersretrievedatabaseinformation.Typically,theuserprovidesastringofcharacters,andthecomputersearchesthedatabaseforacorrespondingsequenceandprovidesthesourcematerialsinwhichthosecharactersappear.Ausercanrequest,forexample,allrecordsinwhichthecontentofthefieldforaperson’slastnameisthewordSmith.
Inflatdatabases,recordsareorganizedaccordingtoasimplelistofentities;manysimpledatabasesforpersonalcomputersareflatinstructure.Therecordsinhierarchicaldatabasesareorganizedinatreelikestructure,witheachlevelofrecordsbranchingoffintoasetofsmallercategories.Unlikehierarchicaldatabases,whichprovidesinglelinksbetweensetsofrecordsatdifferentlevels,networkdatabasescreatemultiplelinkagesbetweensetsbyplacinglinks,orpointers,toonesetofrecordsinanother;thespeedandversatilityofnetworkdatabaseshaveledtotheirwideuseinbusiness.
Relationaldatabasesareusedwhereassociationsamongfilesorrecordscannotbeexpressedbylinks;asimpleflatlistbecomesonetable,or“relation〞,andmultiplerelationscanbemathematicallyassociatedtoyielddesiredinformation.Object-orienteddatabasesstoreandmanipulatemorecomplexdatastructures,called“objects〞,whichareorganizedintohierarchicalclassesthatmayinheritpropertiesfromclasseshigherinthechain;thisdatabasestructureisthemostflexibleandadaptable.
3.StructureoftheRelationaldatabase
Therelationalmodelisthebasisforanyrelationaldatabasemanagementsystem(RDBMS).Arelationalmodelhasthreecorecomponents:
acollectionofobjectsorrelations,operatorsthatactontheobjectsorrelations,anddataintegritymethods.Inotherwords,ithasaplacetostorethedata,awaytocreateandretrievethedata,andawaytomakesurethatthedataislogicallyconsistent.
Arelationaldatabaseusesrelations,ortwo-dimensionaltables,tostoretheinformationneededtosupportabusiness.
3.1.Tables,Row,andColumns
Atableinarelationaldatabase,alternativelyknownasarelation,isatwo-dimensionalstructureusedtoholdrelatedinformation.Adatabaseconsistsofoneormorerelatedtables.
Note:
Don'tconfusearelationwithrelationships.Arelationisessentiallyatable,andarelationshipisawaytocorrelate,join,orassociatetwotables.
Arowinatableisacollectionorinstanceofonething,suchasoneemployeeoronelineitemonaninvoice.Acolumncontainsalltheinformationofasingletype,andthepieceofdataattheintersectionofarowandacolumn,afield,isthesmallestpieceofinformationthatcanberetrievedwiththedatabase'squerylanguage.Forexample,atablewithinformationaboutemployeesmighthaveacolumncalledLAST_NAMEthatcontainsalloftheemployees'lastnames.Dataisretrievedfromatablebyfilteringonboththerowandthecolumn.
3.2.PrimaryKeys,Datatypes,andForeignKeys
Relation:
Atwo-dimensionalstructureusedtoholdrelatedinformation,alsoknownasatable.
Row:
Agroupofoneormoredataelementsinadatabasetablethatdescribesaperson,place,orthing.
Column:
Thecomponentofadatabasetablethatcontainsallofthedataofthesamenameandtypeacrossallrows.
PrimaryKey:
Acolumn(orcolumns)inatablethatmakestherowinthetabledistinguishablefromeveryotherrowinthesametable.
Datatypes:
numericvalues,characteroralphabeticvalues,anddatevalues.
Aforeignkeyenforcestheconceptofreferentialintegrityinarelationaldatabase.
ForeignKey:
Acolumn(orcolumns)inatablethatdrawsitsvaluesfromaprimaryoruniquekeycolumninanothertable.Aforeignkeyassistsinensuringthedataintegrityofatable.ReferentialIntegrityAmethodemployedbyarelationaldatabasesystemthatenforcesone-to-manyrelationshipsbetweentables.
3.3.DataModeling
Inthisprocess,thedeveloperconceptualizesanddocumentsallthetablesforthedatabase.OneofthecommonmethodsformodelingadatabaseiscalledERA,whichstandsforentities,relationships,andattributes.Thedatabasedesignerusesanapplicationthatcanmaintainentities,theirattributes,andtheirrelationships.Ingeneral,anentitycorrespondstoatableinthedatabase,andtheattributesoftheentitycorrespondtocolumnsofthetable.
DataModeling:
Aprocessofdefiningtheentities,attributes,andrelationshipsbetweentheentitiesinpreparationforcreatingthephysicaldatabase.
Thedata-modelingprocessinvolvesdefiningtheentities,definingtherelationshipsbetweenthoseentities,andthendefiningtheattributesforeachoftheentities.Onceacycleiscomplete,itisrepeatedasmanytimesasnecessarytoensurethatthedesigneriscapturingwhatisimportantenoughtogointothedatabase.Let'stakeacloserlookateachstepinthedata-modelingprocess.
3.4.DefiningtheEntities
First,thedesigneridentifiesalloftheentitieswithinthescopeofthedatabaseapplication.
Theentitiesarethepersons,places,orthingsthatareimportanttotheorganizationandneedtobetrackedinthedatabase.Entitieswillmostlikelytranslateneatlytodatabasetables.
3.5.DefiningtheRelationshipsBetweenEntities
Oncetheentitiesaredefined,thedesignercanproceedwithdefininghoweachoftheentitiesisrelated.Often,thedesignerwillpaireachentitywitheveryotherentityandask,"Istherearelationshipbetweenthesetwoentities?
"Somerelationshipsareobvious;somearenot.
Inthewidgetcompanydatabase,thereismostlikelyarelationshipbetweenEMPandDEPT,butdependingonthebusinessrules,itisunlikelythattheDEPTandSALGRADEentitiesarerelated.Ifthebusinessrulesweretorestrictcertainsalarygradestocertaindepartments,therewouldmostlikelybeanewentitythatdefinestherelationshipbetweensalarygradesanddepartments.Thisentitywouldbeknownasanassociativeorintersectiontableandwouldcontainthevalidcombinationsofsalarygradesanddepartments.
AssociativeTable:
Adatabasetablethatstoresthevalidcombinationsofrowsfromtwoothertablesandusuallyenforcesabusinessrule.Anassociativetableresolvesamany-to-manyrelationship.
Ingeneral,therearethreetypesofrelationshipsinarelation