中英文翻译--数据库基础.doc

DatabaseFundamentalsIntroductiontoDBMSAdatabasemanagementsystem(DBMS)isanimportanttypeofprogrammingsystem,usedtodayonthebiggestandthesmallestcomputers.Asforothermajorformsofsystemsoftware,suchascompilersandoperatingsystems,awell-understoodsetofprinciplesfordatabasemanagementsystemshasdevelopedovertheyears,andtheseconceptsareusefulbothforunderstandinghowtousethesesystemseffectivelyandfordesigningandimplementingDBMSs.DBMSisacollectionofprogramsthatenablesyoutostore,modify,andextractinformationfromadatabase.TherearemanydifferenttypesofDBMSs,rangingfromsmallsystemsthatrunonpersonalcomputerstohugesystemsthatrunonmainframes.Therearetwoqualitiesthatdistinguishdatabasemanagementsystemsfromothersortsofprogrammingsystems.1)Theabilitytomanagepersistentdata,and2)Theabilitytoaccesslargeamountsofdataefficiently.Point1)merelystatesthatthereisadatabasewhichexistspermanently；thecontentofthisdatabaseisthedatathataDBMSaccessesandmanages.Point2)distinguishesaDBMSfromafilesystem,whichalsomanagespersistentdata.ADBMSscapabilitiesareneededmostwhentheamountofdataisverylarge,becauseforsmallamountsofdata,simpleaccesstechniques,suchaslinearscansofthedata,areusuallyadequate.WhileweregardtheabovetwopropertiesofaDBMSasfundamental,thereareanumberofothercapabilitiesthatarealmostuniversallyfoundincommercialDBMSs.Theseare:(1)Supportforatleastonedatamodel,ormathematicalabstractionthroughwhichtheusercanviewthedata.(2)Supportforcertainhigh-levellanguagesthatallowtheusertodefinethestructureofdata,accessdata,andmanipulatedata.(3)Transactionmanagement,thecapabilitytoprovidecorrect,concurrentaccesstothedatabasebymanyusersatonce.(4)Accesscontrol,theabilitytolimitaccesstodatabyunauthorizedusers,andtheabilitytocheckthevalidityofdata.(5)Resiliency,theabilitytorecoverfromsystemfailureswithoutlosingdata.DataModelsEachDBMSprovidesatleastoneabstractmodelofdatathatallowstheusertoseeinformationnotasrawbits,butinmoreunderstandableterms.Infact,itisusuallypossibletoseedataatseverallevelsofabstraction.Atarelativelylowlevel,aDBMScommonlyallowsustovisualizedataascomposedoffiles.EfficientFileAccessTheabilitytostoreafileisnotremarkable:thefilesystemassociatedwithanyoperatingsystemdoesthat.ThecapabilityofaDBMSisseenwhenweaccessthedataofafile.Forexample,supposewewishtofindthemanagerofemployeeClarkKent.Ifthecompanyhasthousandsofemployees,ItisveryexpensivetosearchtheentirefiletofindtheonewithNAME=ClarkKent.ADBMShelpsustosetupindexfiles,orindices,thatallowustoaccesstherecordforClarkKentinessentiallyonestrokenomatterhowlargethefileis.Likewise,insertionofnewrecordsordeletionofoldonescanbeaccomplishedintimethatissmallandessentiallyconstant,independentofthefilelength.AnotherthingaDBMShelpsusdoisnavigateamongfiles,thatis,tocombinevaluesintwoormorefilestoobtaintheinformationwewant.QueryLanguagesTomakeaccesstofileseasier,aDBMSprovidesaquerylanguage,ordatamanipulationlanguage,toexpressoperationsonfiles.Querylanguagesdifferinthelevelofdetailtheyrequireoftheuser,withsystemsbasedontherelationaldatamodelgenerallyrequiringlessdetailthanlanguagesbasedonothermodels.TransactionManagementAnotherimportantcapabilityofaDBMSistheabilitytomanagesimultaneouslylargenumbersoftransactions,whichareproceduresoperatingonthedatabase.Somedatabasesaresolargethattheycanonlybeusefuliftheyareoperateduponsimultaneouslybymanycomputers:oftenthesecomputersaredispersedaroundthecountryortheworld.Thedatabasesystemsusebybanks,accessedalmostinstantaneouslybyhundredsorthousandsofautomatedtellermachines(ATM),aswellasbyanequalorgreaternumberofemployeesinthebankbranches,istypicalofthissortofdatabase.Anairlinereservationsystemisanothergoodexample.Sometimes,twoaccessesdonotinterferewitheachother.Forexample,anynumberoftransactionscanbereadingyourbankbalanceatthesametime,withoutanyinconsistency.Butifyouareinthebankdepositingyoursalarycheckattheexactinstantyourspouseisextractingmoneyfromanautomaticteller,theresultofthetwotransactionsoccurringsimultaneouslyandwithoutcoordinationisunpredictable.Thus,transactionsthatmodifyadataitemmust“lockout”othertransactionstryingtoreadorwritethatitematthesametime.ADBMSmustthereforeprovidesomeformofconcurrencycontroltopreventuncoordinatedaccesstothesamedataitembymorethanonetransaction.Evenmorecomplexproblemsoccurwhenthedatabaseisdistributedovermanydifferentcomputersystems,perhapswithduplicationofdatatoallowbothfasterlocalaccessandtoprotectagainstthedestructionofdataifonecomputercrashes.SecurityofDataADBMSmustnotonlyprotectagainstlossofdatawhencrashesoccur,aswejustmentioned,butitmustpreventunauthorizedaccess.Forexample,onlyuserswithacertainclearanceshouldhaveaccesstothesalaryfieldofanemployeefile,andtheDBMSmustbeableassociatewiththevarioususerstheirprivilegestoseefiles,fieldswithinfiles,orothersubsetsofthedatainthedatabase.ThusaDBMSmustmaintainatabletellingforeachuserknowntoit,whataccessprivilegestheuserhasforeachobject.Forexample,oneusermaybeallowedtoreadafile,butnottoinsertordeletedata；anothermaynotbeallowedtoseethefileatall,whileathirdmaybeallowedtoreadormodifythefileatwill.DBMSTypesDesignersdevelopedthreedifferenttypesofdatabasestructures:hierarchical,network,andrelational.Hierarchicalandnetworkwerefirstdevelopedbutrelationalhasbecomedominant.Whiletherelationaldesignisdominant,theolderdatabaseshavenotbeendropped.CompaniesthatinstalledahierarchicalsystemsuchasIMSinthe1970swillbeusingandmaintainingthesedatabasesforyearstocomeeventhoughnewdevelopmentisbeingdoneonrelationalsystems.Theseoldersystemsareoftenreferredtoaslegacysystems.数据库基础DBMS简介数据库管理系统是编程系统中的重要的一种，现今可以用在最大的以及最小的电脑上。其他主要形式的系统软件，比如说汇编以及操作系统，近些年来开发出一系列容易理解的数据库管理系统原则，并且这些概念既有助于理解如何有效利用系统，又可以帮助设计和执行DBMS系统。DBMS是一个程序的集合，它使你能够存储、修改以及从数据库中提取信息。有很多种不同类型的DBMS系统，从运行在个人电脑上的小型系统到运行在大型主机上的巨型系统。DBMS的功能有两种功能使数据库管理系统区别于其他设计系统：1)管理固有数据的能力，以及2)高效访问大量数据的能力。第一点只是表明现有一个固定存在的数据库；而这个数据库的内容也就是DBMS所要访问和管