计算机外文翻译外文文献英文文献数据库系统.docx

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