中英文翻譯--數(shù)據(jù)庫基礎.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“l(fā)ockout”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.數(shù)據(jù)庫基礎DBMS簡介數(shù)據(jù)庫管理系統(tǒng)是編程系統(tǒng)中的重要的一種，現(xiàn)今可以用在最大的以及最小的電腦上。其他主要形式的系統(tǒng)軟件，比如說匯編以及操作系統(tǒng)，近些年來開發(fā)出一系列容易理解的數(shù)據(jù)庫管理系統(tǒng)原則，并且這些概念既有助于理解如何有效利用系統(tǒng)，又可以幫助設計和執(zhí)行DBMS系統(tǒng)。DBMS是一個程序的集合，它使你能夠存儲、修改以及從數(shù)據(jù)庫中提取信息。有很多種不同類型的DBMS系統(tǒng)，從運行在個人電腦上的小型系統(tǒng)到運行在大型主機上的巨型系統(tǒng)。DBMS的功能有兩種功能使數(shù)據(jù)庫管理系統(tǒng)區(qū)別于其他設計系統(tǒng)：1)管理固有數(shù)據(jù)的能力，以及2)高效訪問大量數(shù)據(jù)的能力。第一點只是表明現(xiàn)有一個固定存在的數(shù)據(jù)庫；而這個數(shù)據(jù)庫的內(nèi)容也就是DBMS所要訪問和管