




已閱讀5頁(yè),還剩1頁(yè)未讀, 繼續(xù)免費(fèi)閱讀
版權(quán)說(shuō)明:本文檔由用戶(hù)提供并上傳,收益歸屬內(nèi)容提供方,若內(nèi)容存在侵權(quán),請(qǐng)進(jìn)行舉報(bào)或認(rèn)領(lǐng)
文檔簡(jiǎn)介
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(kù)基礎(chǔ)DBMS簡(jiǎn)介數(shù)據(jù)庫(kù)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(shuō)匯編以及操作系統(tǒng),近些年來(lái)開(kāi)發(fā)出一系列容易理解的數(shù)據(jù)庫(kù)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(shè)計(jì)和執(zhí)行DBMS系統(tǒng)。DBMS是一個(gè)程序的集合,它使你能夠存儲(chǔ)、修改以及從數(shù)據(jù)庫(kù)中提取信息。有很多種不同類(lèi)型的DBMS系統(tǒng),從運(yùn)行在個(gè)人電腦上的小型系統(tǒng)到運(yùn)行在大型主機(jī)上的巨型系統(tǒng)。DBMS的功能有兩種功能使數(shù)據(jù)庫(kù)管理系統(tǒng)區(qū)別于其他設(shè)計(jì)系統(tǒng):1)管理固有數(shù)據(jù)的能力,以及2)高效訪問(wèn)大量數(shù)據(jù)的能力。第一點(diǎn)只是表明現(xiàn)有一個(gè)固定存在的數(shù)據(jù)庫(kù);而這個(gè)數(shù)據(jù)庫(kù)的內(nèi)容也就是DBMS所要訪問(wèn)和管
溫馨提示
- 1. 本站所有資源如無(wú)特殊說(shuō)明,都需要本地電腦安裝OFFICE2007和PDF閱讀器。圖紙軟件為CAD,CAXA,PROE,UG,SolidWorks等.壓縮文件請(qǐng)下載最新的WinRAR軟件解壓。
- 2. 本站的文檔不包含任何第三方提供的附件圖紙等,如果需要附件,請(qǐng)聯(lián)系上傳者。文件的所有權(quán)益歸上傳用戶(hù)所有。
- 3. 本站RAR壓縮包中若帶圖紙,網(wǎng)頁(yè)內(nèi)容里面會(huì)有圖紙預(yù)覽,若沒(méi)有圖紙預(yù)覽就沒(méi)有圖紙。
- 4. 未經(jīng)權(quán)益所有人同意不得將文件中的內(nèi)容挪作商業(yè)或盈利用途。
- 5. 人人文庫(kù)網(wǎng)僅提供信息存儲(chǔ)空間,僅對(duì)用戶(hù)上傳內(nèi)容的表現(xiàn)方式做保護(hù)處理,對(duì)用戶(hù)上傳分享的文檔內(nèi)容本身不做任何修改或編輯,并不能對(duì)任何下載內(nèi)容負(fù)責(zé)。
- 6. 下載文件中如有侵權(quán)或不適當(dāng)內(nèi)容,請(qǐng)與我們聯(lián)系,我們立即糾正。
- 7. 本站不保證下載資源的準(zhǔn)確性、安全性和完整性, 同時(shí)也不承擔(dān)用戶(hù)因使用這些下載資源對(duì)自己和他人造成任何形式的傷害或損失。
最新文檔
- 高中英語(yǔ)學(xué)術(shù)詞匯教學(xué)與口語(yǔ)表達(dá)能力提升策略論文
- 節(jié)約用蒸汽管理制度
- 茶餐廳消防管理制度
- 財(cái)務(wù)管理學(xué)及財(cái)務(wù)知識(shí)課后分析答案(一)
- 管理心理學(xué)案例分析-摩托羅拉公司的內(nèi)部激勵(lì)
- 自動(dòng)化儀表與監(jiān)測(cè)在工業(yè)中變革與創(chuàng)新
- 財(cái)稅實(shí)務(wù)企業(yè)并購(gòu)重組的企業(yè)所得稅與土地增值稅政策比較分析
- 2025年重慶一中中考數(shù)學(xué)三模試卷
- 設(shè)備綜合管理制度范本大全
- 財(cái)務(wù)會(huì)計(jì)實(shí)訓(xùn)個(gè)人心得體會(huì)10篇
- 模板施工方案 加油站
- 質(zhì)量管理體系變更管理制度
- 硫化氫中毒現(xiàn)場(chǎng)處置方案
- 系統(tǒng)集成方案及實(shí)施步驟
- 2025年隴南村文書(shū)考試題及答案
- 2025年中科院心理咨詢(xún)師培訓(xùn)考試復(fù)習(xí)題庫(kù)-上(單選題)
- ?;?jí)安全教育
- 馬克思主義基本原理與科技創(chuàng)新的結(jié)合心得體會(huì)
- 美發(fā)店投資入股協(xié)議書(shū)8篇
- 第四單元 課題3 物質(zhì)組成的表示教學(xué)設(shè)計(jì)-2024-2025學(xué)年九年級(jí)化學(xué)人教版(2024)上冊(cè)
- DeepSeek零基礎(chǔ)到精通手冊(cè)(保姆級(jí)教程)
評(píng)論
0/150
提交評(píng)論