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中英文翻译--数据库基础.doc

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中英文翻译--数据库基础.doc

DatabaseFundamentalsIntroductiontoDBMSAdatabasemanagementsystemDBMSisanimportanttypeofprogrammingsystem,usedtodayonthebiggestandthesmallestcomputers.Asforothermajorformsofsystemsoftware,suchascompilersandoperatingsystems,awellunderstoodsetofprinciplesfordatabasemanagementsystemshasdevelopedovertheyears,andtheseconceptsareusefulbothforunderstandinghowtousethesesystemseffectivelyandfordesigningandimplementingDBMSs.DBMSisacollectionofprogramsthatenablesyoutostore,modify,andextractinformationfromadatabase.TherearemanydifferenttypesofDBMSs,rangingfromsmallsystemsthatrunonpersonalcomputerstohugesystemsthatrunonmainframes.Therearetwoqualitiesthatdistinguishdatabasemanagementsystemsfromothersortsofprogrammingsystems.1Theabilitytomanagepersistentdata,and2Theabilitytoaccesslargeamountsofdataefficiently.Point1merelystatesthatthereisadatabasewhichexistspermanentlythecontentofthisdatabaseisthedatathataDBMSaccessesandmanages.Point2distinguishesaDBMSfromafilesystem,whichalsomanagespersistentdata.ADBMSscapabilitiesareneededmostwhentheamountofdataisverylarge,becauseforsmallamountsofdata,simpleaccesstechniques,suchaslinearscansofthedata,areusuallyadequate.WhileweregardtheabovetwopropertiesofaDBMSasfundamental,thereareanumberofothercapabilitiesthatarealmostuniversallyfoundincommercialDBMSs.Theseare1Supportforatleastonedatamodel,ormathematicalabstractionthroughwhichtheusercanviewthedata.2Supportforcertainhighlevellanguagesthatallowtheusertodefinethestructureofdata,accessdata,andmanipulatedata.3Transactionmanagement,thecapabilitytoprovidecorrect,concurrentaccesstothedatabasebymanyusersatonce.4Accesscontrol,theabilitytolimitaccesstodatabyunauthorizedusers,andtheabilitytocheckthevalidityofdata.5Resiliency,theabilitytorecoverfromsystemfailureswithoutlosingdata.DataModelsEachDBMSprovidesatleastoneabstractmodelofdatathatallowstheusertoseeinformationnotasrawbits,butinmoreunderstandableterms.Infact,itisusuallypossibletoseedataatseverallevelsofabstraction.Atarelativelylowlevel,aDBMScommonlyallowsustovisualizedataascomposedoffiles.EfficientFileAccessTheabilitytostoreafileisnotremarkablethefilesystemassociatedwithanyoperatingsystemdoesthat.ThecapabilityofaDBMSisseenwhenweaccessthedataofafile.Forexample,supposewewishtofindthemanagerofemployeeClarkKent.Ifthecompanyhasthousandsofemployees,ItisveryexpensivetosearchtheentirefiletofindtheonewithNAMEClarkKent.ADBMShelpsustosetupindexfiles,orindices,thatallowustoaccesstherecordforClarkKentinessentiallyonestrokenomatterhowlargethefileis.Likewise,insertionofnewrecordsordeletionofoldonescanbeaccomplishedintimethatissmallandessentiallyconstant,independentofthefilelength.AnotherthingaDBMShelpsusdoisnavigateamongfiles,thatis,tocombinevaluesintwoormorefilestoobtaintheinformationwewant.QueryLanguagesTomakeaccesstofileseasier,aDBMSprovidesaquerylanguage,ordatamanipulationlanguage,toexpressoperationsonfiles.Querylanguagesdifferinthelevelofdetailtheyrequireoftheuser,withsystemsbasedontherelationaldatamodelgenerallyrequiringlessdetailthanlanguagesbasedonothermodels.TransactionManagementAnotherimportantcapabilityofaDBMSistheabilitytomanagesimultaneouslylargenumbersoftransactions,whichareproceduresoperatingonthedatabase.Somedatabasesaresolargethattheycanonlybeusefuliftheyareoperateduponsimultaneouslybymanycomputersoftenthesecomputersaredispersedaroundthecountryortheworld.Thedatabasesystemsusebybanks,accessedalmostinstantaneouslybyhundredsorthousandsofautomatedtellermachinesATM,aswellasbyanequalorgreaternumberofemployeesinthebankbranches,istypicalofthissortofdatabase.Anairlinereservationsystemisanothergoodexample.Sometimes,twoaccessesdonotinterferewitheachother.Forexample,anynumberoftransactionscanbereadingyourbankbalanceatthesametime,withoutanyinconsistency.Butifyouareinthebankdepositingyoursalarycheckattheexactinstantyourspouseisextractingmoneyfromanautomaticteller,theresultofthetwotransactionsoccurringsimultaneouslyandwithoutcoordinationisunpredictable.Thus,transactionsthatmodifyadataitemmustlockoutothertransactionstryingtoreadorwritethatitematthesametime.ADBMSmustthereforeprovidesomeformofconcurrencycontroltopreventuncoordinatedaccesstothesamedataitembymorethanonetransaction.Evenmorecomplexproblemsoccurwhenthedatabaseisdistributedovermanydifferentcomputersystems,perhapswithduplicationofdatatoallowbothfasterlocalaccessandtoprotectagainstthedestructionofdataifonecomputercrashes.SecurityofDataADBMSmustnotonlyprotectagainstlossofdatawhencrashesoccur,aswejustmentioned,butitmustpreventunauthorizedaccess.Forexample,onlyuserswithacertainclearanceshouldhaveaccesstothesalaryfieldofanemployeefile,andtheDBMSmustbeableassociatewiththevarioususerstheirprivilegestoseefiles,fieldswithinfiles,orothersubsetsofthedatainthedatabase.ThusaDBMSmustmaintainatabletellingforeachuserknowntoit,whataccessprivilegestheuserhasforeachobject.Forexample,oneusermaybeallowedtoreadafile,butnottoinsertordeletedataanothermaynotbeallowedtoseethefileatall,whileathirdmaybeallowedtoreadormodifythefileatwill.DBMSTypesDesignersdevelopedthreedifferenttypesofdatabasestructureshierarchical,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所要访问和管理的那些数据。第二点将DBMS和同样能管理固有数据的文件系统区分开来。通常在数据量非常大的时候才需要用到DBMS系统的功能,因为对于小量数据而言,简单的访问技术(如对数据的线性扫描)就足够了。虽然我们将以上两点作为DBMS的基本特性,但是其他一些功能也是在商业DBMS系统中常见的,它们是(1)支持至少一种用户可以据之浏览数据的数据模式或数学提取方式。(2)支持某种允许用户用来定义数据的结构、访问和操纵数据的高级语言。(3)事务管理,即对多个用户提供正确、同时访问数据库的能力。(4)访问控制,即限制未被授权用户对数据的访问能力,以及检测数据有效性的能力。(5)恢复功能,即能够从系统错误中恢复过来而不丢失数据的能力。数据模型每个DBMS提供了至少一种允许用户不是以原始比特位的方式,而是以更容易理解的术语来观看信息的抽象数据模型。实际上,通常要观察以几个不同级别提取出来的数据是可能的。在相关的低级别中,DBMS一般允许我们将数据形象化为文件的组成部分。高效数据访问存储一个文件的能力并不特别操作系统中结合的文件系统都能够如此。DBMS的能力在我们访问文件的数据时才能显露出来。比如,假设我们希望找到员工经理克拉克肯特。如果这个公司有上万员工,则要通过NAME=克拉克肯特搜索整个文件来找到这个人是非常费时的。而DBMS帮助我们建立索引文件或索引,不管文件有多大,它都使我们能够一举访问到克拉克肯特的记录。同样的,新记录的插入或者原有记录的删除都可以在较

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