calStructures.Peoplevisualizeorstructuredatainlogicalwaysfortheirownpurposes.Thus,recordsR1andR2mayalwaysbelogicallylinkedandprocessedinsequenceinoneparticularapplication.However,inacomputersystemit'squitepossiblethattheserecordsthatarelogicallycontiguousinoneapplicationarenotphysicallystoredtogether.Rather,thephysicalstructureoftherecordsinmediaandhardwaremaydependnotonlyontheI/Oandstoragedevicesandtechniquesused,butalsoonthedifferentlogicalrelationshipsthatusersmayassigntothedatafoundinR1andR2.Forexample,R1andR2mayberecordsofcreditcustomerswhohaveshipmentssendtothesameblockinthesamecityevery2weeks.Fromtheshippingdepartmentmanager'sperspective,then,R1andR2aresequentialentriesonageographicallyorganizedshippingreport.ButintheA/Rapplication,thecustomersrepresentedbyR1andR2maybeidentified,andtheiraccountsmaybeprocessed,accordingtotheiraccountnumberswhicharewidelyseparated.Inshort,then,thephysicallocationofthestoredrecordsinmanycomputer-basedinformationsystemsisinvisibletousers.
12-3DATAINDEPENDENCE,INTEGRITYANDSECURITY
DATAINDEPENDENCE
Animportantpointaboutdatabasesystemsisthatthedatabaseshouldexistindependentlyofanyofthespecificapplications.Traditionaldataprocessingapplicationsaredatadependent.COBOLprogramscontainfiledescriptionsandrecorddescriptionsthatcarefullydescribetheformatandcharacteristicsofthedata.
Usersshouldbeabletochangethestructureofthedatabasewithoutaffectingtheapplicationsthatuseit.Forexample,supposethattherequirementsofyourapplicationschange.AsimpleexamplewouldbeexpandingZIPcodesfromfivedigitstoninedigits.InatraditionalapproachusingCOBOLprograms,eachindividualCOBOLapplicationprogramthatusedthatparticularfieldwouldhavetobechanged,recompiled,andretested.Theprogramswouldbeunabletorecognizeoraccessafilethathadbeenchangedandcontainedanewdatadescription;this,inturn,mightcausedisruptioninprocessingunlessthechangewerecarefullyplanned.
MostdatabaseprogramsprovidetheabilitytochangethedatabasestructurebysimplychangingtheZIPcodefieldandthedata-entryform.Inthiscase,dataindependenceallowsforminimaldisruptionofcurrentandexistingapplications.Userscancontinuetoworkandcanevenignorethenine-digitcodeiftheychoose.Eventually,thefilewillbeconvertedtothenewnine-digitZIPcode,buttheeasewithwhichthechangeovertakesplaceemphasizestheimportanceofdataindependence.
DATAINTEGRITY
Dataintegrityreferstotheaccuracy,correctness,orvalidityofthedatainthedatabase.Inadatabasesystem,dataintegritymeanssafeguardingthedataagainstinvalidalterationordestruction.Inlargeon-linedatabasesystem,dataintegritybecomesamoresevereproblemandtwoadditionalcomplicationsarise.Thefirsthastodowithmanyusersaccessingthedatabaseconcurrently.Forexample,ifthousandsoftravelagentsandairlinereservationclerksareaccessingthesamedatabaseatonce,andtwoagentsbookthesameseatonthesameflight,thefirstagent'sbookingwillbelost.Insuchcasesthetechniqueoflockingtherecordorfieldprovidesthemeansforpreventingoneuserfromaccessingarecordwhileanotheruserisupdatingthesamerecord.
Thesecondcomplicationrelatestohardware,software,orhumanerrorduringthecourseofprocessingandinvolvesdatabasetransactionstokeepthedatabaseinaconsistentstateofintegrity.Adatabasetransactionisagroupofdatabasemodificationstreatedasasingleunit.Forexample,anagentbookinganairlinereservationinvolvesseveraldatabaseupdates(i.e.,addingthepassenger'snameandaddressandupdatingtheseats-availablefield),whichcompriseasingletransaction.Thedatabasetransactionisnotconsideredtobecompleteduntilallupdateshavebeencompleted;otherwise,noneoftheupdateswillbeallowedtotakeplace.
DATASECURITY
Datasecurityreferstotheprotectionofadatabaseagainstunauthorizedorillegalaccessormodification.Thisusuallyinvolvesoneormorelevelsofpasswordprotectionthatarespecifiedinthedatadictionary.Forexample,ahigh-levelpasswordmightallowausertoreadfrom,writeto,andmodifythedatabasestructure,whereasalow-levelpasswordmightonlyallowausertoreadfromthedatabase.
Oftenanaudittrail-therecordedhistoryofthemodificationstoadatabase-canbeusedtoidentifywhereandwhenadatabasewastamperedwithanditcanalsobeusedtorestorethefiletoitsoriginalcondition.
12-4MANAGEMENTINFORMATIONSYSTEM(MIS)
Themanagementinformationsystem(MIS)concepthasbeendefinedindozensofways.Sinceoneorganization'smodelofanMISislikelytodifferfromthatofanother,it'snotsurprisingthattheirMISdefinitionswouldalsovaryinscopeandbreadth.Forourpurposes,anMIScanbedefinedasanetworkofcomputer-baseddataprocessingproceduresdevelopedinanorganizationandintegratedasnecessarywithmanualandotherproceduresforthepurposeofprovidingtimelyandeffectiveinformationtosupportdecisionmakingandothernecessarymanagementfunctions.
AlthoughMISmodelsdiffer,mostofthemrecognizetheconceptsshowninfig12-6.Inadditiontowhatmightbetermedthehorizontalmanagementstructureshowninfig12-6(a),anorganizationisalsodividedverticallyintodifferentspecialitiesandfunctionswhichrequireseparateinformationflows(seefig12-6(b)).Combiningthehorizontalmanageriallevelswiththeverticalspecialitiesproducesthecomplexorganizationalstructureshowninfig12-6(c).Underlyingthisstructureisadatabaseconsisting,idealy,ofinternallyandexternallyproduceddatarelatingtopast,present,andpredictedfurtureevents.
TheformidabletaskoftheMISdesigneristodeveloptheinformationflowneededtosupportdecisionmaking(seefig12-7).Generallyspeaking,muchoftheinformationneededbymanagerswhooccupydifferentlevelsandwhohavedifferentresponsibilitiesisobtainedfromacollectionofexistinginformationsystems(orsubsystems).ThesesystemsmaybetiedtogetherverycloselyinanMIS.Moreoften,however,theyaremorelooselycoupled.
中文翻译2:
数据库系统
12-1数据库管理系统(DBMS)
众所周知,数据库是逻辑上相关的数据元的集合。这些数据元可以按不同的结构组织起来,以满足单位和个人的多种处理和检索的需要。数据库本身不是什么新鲜事--早期的数据库凿在石头上,记在名册上,以及写入索引卡中。而现在,数据库普遍记录在可磁化的介质上,并且需要用计算机程序来执行必需的存储和检索操作。
在后文中你将看到除了最简单的以外,所有数据库中
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