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.數據庫基礎DBMS簡介數據庫管理系統是編程系統中的重要的一種，現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軟件，比如說匯編以及操作系統，近些年來開發出一系列容易理解的數據庫管理系統原則，并且這些概念既有助于理解如何有效利用系統，又可以幫助設計和執行DBMS系統。DBMS是一個程序的集合，它使你能夠存儲、修改以及從數據庫中提取信息。有很多種不同類型的DBMS系統，從運行在個人電腦上的小型系統到運行在大型主機上的巨型系統。DBMS的功能有兩種功能使數據庫管理系統區別于其他設計系統：1)管理固有數據的能力，以及2)高效訪問大量數據的能力。第一點只是表明現有一個固定存在的數據庫；而這個數據庫的內容也就是DBMS所要訪問和管