外文翻译 - 对于数据库系统的介绍

外文资料AnIntroductiontoDatabaseSystemsADatabaseManagementSystem(DBMS)consistsofacollectionofinterrelateddataandasetofprogramstoaccessthosedata.Adatabaseisacollectionofdataorganizedtoservermanyapplicationsefficientlybycentralizingthedataandminimizingredundantdata.TheprimarygoalofaDBMSistoprovideanenvironmentthatisbothconvenientandefficienttouseinretrievingandstoringdatabaseinformation.Databasesystemsaredesignedtomanagelargebodiesofinformation.Themanagementofdatainvolvesboththedefinitionofstructuresforthestorageofinformationandtheprovisionofmechanismsforthemanipulationofinformationstored,despitesystemmustavoidpossibleanomalousresults.Theimportantofinformationinmostorganizations,whichdeterminesthevalueofthedatabase,hasledtothedevelopmentofalargebodyofconceptsandtechniquesforthedatabase,hasledtothedevelopmentofalargebodyofconceptsandtechniquesfortheefficientmanagementofdata.Thetypicalfile-processingsystemissupportedbyaconventionaloperatingsystem.Permanentrecordsarestoredinvariousfiles,anddifferentapplicationprogramsarewrittentoextractrecordsfrom,andtoaddrecordsto,theappropriatefiles.BeforetheadventofDBMSs,organizationstypicallystoredinformationusingsuchsystems.Keepingorganizationalinformationinafile-processingsystemhasanumberofmajordisadvantages.Dataredundancyandinconsistency.Dataredundancyisthepresenceofduplicatedatainmultipledatafiles.Sincethefilesandapplicationprogramsarecreatedbydifferentprogrammersoveralongperiod,thevariousfilesarelikelytohavedifferentformatsandtheprogramsmaybewritteninseveralprogramminglanguages.Moreover,thesameinformationmaybeduplicatedinseveralplaces(files).Thisredundancyleadstohigherstorageandaccesscost.Inaddition,itmayleadtodatainconsistency;thatis,thevariouscopiesofthesamedatamaynolongeragree.Difficultyinaccessingdata.Thepointhereisthatconventionalfile-processingenvironmentsdonotallowneededdatatoberetrievedinaconvenientandefficientmanner.Moreresponsivedata-retrievalsystemsmustbedevelopedforgeneraluse.Integrityproblems.Thedatavaluesstoredinthedatabasemustsatisfycertaintypesofconsistencyconstrains.Developersenforcetheconstraintsinthesystembyaddingappropriatecodeinthevariousapplicationprograms.However,whennewconstraintsareadded,itisdifficulttochangetheprogramstoenforcethem.Theproblemiscompoundedwhenconstraintsinvolveseveraldataitemsfromdifferentfiles.Atomicityproblems.Acomputersystem,likeanyothermechanicalorelectricaldevice,issubjecttofailure.Inmanyapplications,itiscrucialtoensurethat,onceafailurehasoccurredandhasbeendetected,thedataarerestoredtotheconsistentstatethatexistedpriortothefailure.Consideraprogramtotransfer$50fromaccountAtoB.Ifasystemfailureoccursduringtheexecutionoftheprogram,itispossiblethatthe$50wasremovedfromaccountAbutwasnotcreditedtoaccountB,resultinginaninconsistentdatabasestate.Clearly,itisessentialtodatabaseconsistencythateitherboththecreditanddebitoccur,orthatneitheroccurs.Thatis,thefundstransfermustbeatomic-itmusthappeninitsentiretyornotatall.Itisdifficulttoensurethispropertyinaconventionalfile-processingsystem.Concurrent-accessanomalies.Sothattheoverallperformanceofthesystemisimprovedandafasterresponsetimeispossible,manysystemsallowmultipleuserstoupdatethedatasimultaneously.Insuchanenvironment,interactionofconcurrentupdatesmayresultininconsistentdata.ConsiderbankaccountA,containing$500.IftwocustomerswithdrawfundsfromaccountAataboutthesametime,theresultoftheconcurrentexecutionsmayleavetheaccountinanincorrectstate.Supposethattheprogramsexecutingonbehalfofeachwithdrawalreadtheoldbalance,reducethatvaluebytheamountbeingwithdraw,andwritetheresultback.Ifthetwoprogramsrunconcurrently,theymaybothreadthevalue$500,andwriteback$450and$400,respectively.Dependingonwhichonewritesthevaluelast,theaccountmaycontaineither$450or$400,ratherthanthecorrectvalueof$350.Toguardagainstthispossibility,thesystemmustmaintainsomeformofsupervision.Becausedatamaybeaccessedbymanydifferentapplicationprogramsthathavenotbeencoordinatedpreviously,however,supervisionisdifficulttoprovide.Securityproblems.Noteveryuserofthedatabasesystemshouldbeabletoaccessallthedata.Forexample,inabankingsystem,payrollpersonnelneedtoseeonlythatpartofthedatabasethathasinformationaboutthevariousbankemployees.Theydonotneedaccesstoinformationaboutcustomeraccounts.Sinceapplicationprogramsareaddedtothesysteminanadhocmanner,itisdifficulttoenforcesuchsecurityconstraints.Thesedifficulties,amongothers,havepromptedthedevelopmentofDBMSs.Inwhatfollows,weshallseetheconceptsandalgorithmsthathavebeendevelopedfordatabasesystemstosolvetheproblemsmentioned.Atypicaldata-processingapplicationstoresalargenumberofrecords,eachofwhichisfairlysimpleandsmall.ADBMSisacollectionofinterrelatedfilesandasetofprogramsthatallowuserstoaccessandmodifythesefiles.Amajorpurposeofadatabasesystemistoprovideuserswithanabstractviewofthedata.Thatis,thesystemhidescertaindetailsofhowthedataarestoredandmaintained.Forthesystemtobeusable,itmustretrievedataefficiently.Thisconcernhasledtothedesignofcomplexityfromusersthroughseverallevelsofabstraction,tosimplifyusersinteractionswiththesystem:A:Physicallevels.Thelowestlevelofabstractiondescribeshowthedataareactuallystored.Atthephysicallevel,complexlow-leveldatastructuresaredescribedindetail.B:Logicallevel.Thenext-higherlevelofabstractiondescribeswhatdataarestoredindatabase,andwhatrelationshipsexistamongthosedata.Theentiredatabaseisthusdescribedintermsofasmallnumberofrelativelysimplestructures.Althoughimplementationofthesimplestructuresatthelogicallevelmayinvolvecomplexphysical-levelstructures,theuserofthelogicalleveldoesnotneedtobeawareofthiscomplexity.Thelogicallevelofabstractionisusedbydatabaseadministrators,whomustdecidewhatinformationistobekeptinthedatabase.C:Viewlevel.Thehighestlevelofabstractiondescribesonlypartoftheentiredatabase.Despitetheuseofsimplerstructuresatthelogicallevel,somecomplexityremains,becauseofthelargesizeofthedatabase.Manyusersofthedatabasesystemwillnotbeconcernedwithallthisinformation.Instead,suchusersneedtoaccessonlyapartofthedatabase.Sothattheirinteractionwiththesystemissimplified,theviewlevelofabstractionisdefined.Thesystemmayprovidemanyviewsforthesamedatabase.Theabilitytomodifyaschemadefinitioninonelevelwithoutaffectingaschemadefinitioninthenexthigherleveliscalleddataindependence.Therearetwolevelsofdataindependence:Physicaldataindependenceistheabilitytomodifythephysicalschemawithoutcausingapplicationprogramstoberewritten.Modificationsatthephysicallevelareoccasionallynecessarytoimproveperformance.Logicaldataindependenceistheabilitytomodifythelogicalschemawithoutcausingapplicationprogramstoberewritten.Modificationsatthelogicallevelarenecessarywheneverthelogicalstructureofthedatabaseisaltered.Logicaldataindependenceismoredifficulttoachievethanphysicaldataindependence,sinceapplicationprogramsareheavilydependentonthelogicalstructureofthedatathattheyaccess.Theconceptofdataindependenceissimilarinmanyrespectstotheconceptofabstractdatatypesinmodemprogramminglanguages.Bothhideimplementationdetailsfromtheusers,toallowuserstoconcentrateonthegeneralstructure,ratherthanonlow-levelimplementationdetails.Thestructuredquerylanguage(SQL)isthemostwidelyusedandstandardquerylanguageforrelationaldatabasemanagementsystems.Itisakindofnon-procedurallanguage.TheSQLlanguagehasseveralparts:Data-definitionlanguage(DDL).TheSQLDDLprovidescommandsfordefiningrelationschemas,deletingrelations,creatingindices,andmodifyingrelationschemas.Interactivedata-manipulationlanguage(DML).TheSQLDMLincludeaquerylanguagebasedonboththerelationalalgebraandthetuplerelationalcalculus.Itincludesalsocommandstoinserttuplesinto,deletetuplesfrom,andmodifytuplesinthedatabase.EmbeddedDML.TheembeddedformofSQLisdesignedforusewithingeneral-purposeprogramminglanguages.Viewdefinition.TheSQLDDLincludecommandsfordefiningviews.Authorization.TheSQLDDLincludescommandsforspecifyingaccessrightstorelationsandviews.Integrity.TheSQLDDLincludescommandsforspecifyingintegrityconstraintsthatthedatastoredinthedatabasemustsatisfy.Updatesthatviolateintegrityconstraintsaredisallowed.Transactioncontrol.SQLincludescommandsforspecifyingthebeginningandendingoftransactions.Severalimplementationsalsoallowexplicitlockingofdataforconcurrencycontrol.Atransactionisacollectionofoperationsthatperformsasinglelogicalfunctioninadatabaseapplication.Eachtransactionisaunitofbothatomicityandconsistency.Thus,werequirethattransactiondonotviolateanydatabaseconsistencyconstraints.Thatis,ifthedatabasewasconsistentwhenatransactionstarted,thedatabasemustbeconsistentwhenthetransactionsuccessfullyterminates.However,duringtheexecutionofatransaction,itmaybenecessarytemporarilytoallowinconsistency.Thistemporaryinconsistency,althoughnecessary,mayleadtodifficultyifafailureoccurs.中文翻译对于数据库系统的介绍一个数据库管理系统由一些相关的数据和一组用来访问那些数据的程序组成。一个数据库是一个为了有效地服务各种应用软件的数据组织的集合通过集中并减少数据的冗余。数据库管理系统最主要的一个目标就是去提供一个既方便又有效地查询、存储数据信息系统的环境。数据库系统被设计用来管理拥有很大数据量的信息。这些数据的管理包括存储信息结构的定义和处理存储信息的规范机制，尽管系统必须避免可能的不规则的结果。在大多数的机构里信息有着重要性，这点决定了数据库的价值，让数据库的观念和技术有了很大的发展，让有效的数据管理观念和技术有了很大的发展。典型的文件处理系统被传统的操作系统支持。永久性的记录存储在各种各样的文件中，并且，不同应用程序被编写用来读取记录和添加记录在恰当得文件里。在数据库管理系统出现以前，组织代表性的信息存储用那些系统来实现。在文件处理系统中保持组织的信息有许多不利得条件。（1）数据冗余和矛盾。数据冗余是完全相同的数据存在于若干数据文件中。既然文件和应用程序被不同的编程员创建在很长的一个周期里，各种各样的文件很可能有着不同的格式和程序，可能用不同的编程语言编写。此外，相同的信息可能在一些文件里被复制出来。这种冗余导致数据矛盾；更确切的说，各种各样的相同数据的复制不再一致。（2）访问数据的困难。这一点是关于传统的文件处理环境不允许被需求的数据通过一种方便和有效地方式被找回。更多的响应得数据检索系统必须被开发用来普遍使用。（3）完整性的问题。数据有价值的存储在数据库中必须满足强迫连接的确定类型。开发者坚持系统中的约束通过增加适当的编码在各种各样的应用程序中。然而，当新的约束被添加改变程序去执行它们是困难的。当约束涉及不同文件中的一些数据项，这种问题是复合的。（4）原子数问题。一个计算机系统，想起他的机械的或电子的装置常遭受失败。在许多应用软件中，保证它的安全性是最重要的，一旦故障发生并且已经被发觉，数据被先前的故障还原保持一致状态。考虑到一个程序传递50美元从总账A到B。如果一个系统故障发生在程序的执行期间，它可能从总账A转移50美元，但是并没有记入到总账B，结果是造成了不一致的数据库状态。明显地，无论是贷方或借方发生，还是两者都不发生，有必要去保持数据库的一致性。更确切的说，基金过户必须是原子的-它必须全部发生或完全不发生。确保这种性质在传统的文件处理系统实现时不可能的。（5）不规则的同时访问。因此，系统的全部工作特性被改良并且实现一个更快的反应时间是可能的。许多系统允许多重用户同时更新系统。在这种环境下，同时更新资料的交互发生可能导致产生不一致数据。考虑到，银行总账A包含500美元。如果两个消费者同时从总账A撤销基金，这样同时发生的事件导致的结果是执行可能是错误的状态。假如正在执行的程序代表每一个取消读取旧的结余，通过总量被撤回减少它们的价值，并且写回结果。如果这两个程序同时运行，它们可能既读取价值500美元，又分别得写回450美元和400美元。依靠最后写入价值的信息，总账可能包含或是450美元或是400美元，而不是正确的价值350美元。为了预防这种可能发生的情况，系统必须维持一些监督形式。因为数据可能被访问通过以前没有被调整的许多不同的应用程序，然而，提供监督机制是很难被实现的。（6）安全问题。并不是每一个数据库系统的用户应该能够访问所有的数据。例如，在一个银行系统，职员的薪水册，需要只能看到数据库系统的一部分，那些包含各类银行职员的信息。他们不需要访问关于消费者账号的信息。既然，应用软件程序用一种特别的方式添加到系统中，去执行那种安全约束是很困难的一件事情。这些困难，除了别的以外，还有即时的数据库管理系统的发展。在下文中，我们将会了解为了数据库系统而发展起来的有关概念和运算法则，去解决涉及到得问题。一个典型的数据处理应用程序存储着相当多的记录，他们中的每一个都相当的简洁。一个数据库管理系统是一个收集相关文件和允许用户访问和修改那些文件的一组程序。数据库系统的主要目的是给用户提供一些关于数据观点的摘要。更确切的说，系统隐藏着特定的详细资料关于数据怎样存储和维持。为了使系统可用，它必须有效地找回数据。这个关系到复杂事物的设计从使用者通过各种结构级别，去简单化用户与系统的交互作用：A：物理级别。是结构描述的最低级别。关于数据怎样实际存储。在物理级别里，综合的低水平数据结构详细的被描述。B：逻辑级别。结构更高一级的级别描述着什么数据被存储在数据库中