Mode
Definition:Alsocalledlogicalmode,itisthedescriptionofthelogicalstructureandcharacteristicsofalldatainthedatabase,andisthecommondataviewforallusers.
Understanding:
①Adatabasehasonlyonemode;
②Itistheviewofdatabasedataatthelogicallevel;
③DatabaseThemodeisbasedonacertaindatamodel;
④Whendefiningthemode,itisnotonlynecessarytodefinethelogicalstructureofthedata(suchaswhichdataitemsconstitutethedatarecord,thename,type,valuerangeofthedataitem,etc.),Anddefinethesecurityandintegrityrequirementsrelatedtothedata,anddefinetherelationshipbetweenthesedata.
ExternalSchema
Definition:alsoknownasSubschemaorusermode,whichcanbeseenandusedbydatabaseusers(includingapplicationprogrammersandendusers)Thedescriptionofthelogicalstructureandcharacteristicsofthelocaldataisthedataviewofthedatabaseuserandthelogicalrepresentationofthedatarelatedtoacertainapplication.
Understanding:
①Adatabasecanhavemultipleexternalmodes;
②Theexternalmodeistheuserview;
③TheexternalmodeItisapowerfulmeasuretoensuredatasecurity.
InternalSchema
Definition:alsoknownasStorageSchema,itisthedescriptionofthephysicalstructureandstorageofdata,andtherepresentationofdatainthedatabase(Forexample,isthestoragemethodofrecordsstoredsequentially,storedinaB-treestructure,orstoredinahashmethod;howtheindexisorganized;whetherthedataiscompressedandstoredandwhetheritisencrypted;whataretherequirementsforthestoragerecordstructureofthedata).
Understanding:
①Adatabasehasonlyoneinternalschema;
②Atablemayconsistofmultiplefiles,suchasdatafilesandindexfiles.
Itisamethodforthedatabasemanagementsystem(DBMS)toeffectivelyorganizeandmanagethedatainthedatabase
Itspurposeis:
①Inordertoreducedataredundancy,Realizedatasharing;
②Inordertoimproveaccessefficiencyandperformance.
SchemaInterpretation(ConceptualSchema)
ConceptualSchemaisthedescriptionoftheglobaldatalogicalstructureinthedatabasesystem,andisthecommondataviewofallusers(applications).ThisdescriptionisanabstractionThedescriptionof,itdoesnotinvolvespecifichardwareenvironmentandplatform,andhasnothingtodowithspecificsoftwareenvironment.
Theconceptualmodelmainlydescribestheconceptualrecordtypeofdataandthedataandtherelationshipbetweenthem.Italsoincludessomesemanticconstraintsbetweenthedata.ItsdescriptioncanbedefinedbytheDDLlanguageintheDBMS.
ExternalSchema
ExternalSchemaisalsocalledSubschemaorUser'sschema.Itistheuser’sdataview,thatis,theuser’sApartoftheseenpatternisderivedfromtheconceptualpattern.Theconceptualpatterngivesaglobaldatadescriptionofthesystemandtheouterpatterngivesalocaldescriptionofeachuser.Aconceptualmodecanhaveseveralexternalmodes,andeachuseronlycaresaboutthemoderelatedtoit.Thiscanshieldalargeamountofirrelevantinformationandisbeneficialtodataprotection,soitisextremelybeneficialtousers.IngeneralDBMS,therearerelatedexternalmodedescriptionlanguages(externalmodeDDL).
InternalSchema
InternalSchemaisalsocalledPhysicalSchema,whichgivesDatabasephysicalstoragestructureandphysicalaccessmethods,suchasdatastoragefilestructure,index,clusteringandhashaccessmethodsandaccesspaths.Thephysicalityoftheinternalmodeismainlyreflectedintheoperatingsystemandfilelevel.Deepdowntothedevicelevel(suchasdiskanddiskoperations),butinrecentyearstherehasbeenatrendtowardthedevicelevel(suchasrawdisk,diskblocktechnology,etc.),DBMSgenerallyprovidesrelatedinternalmodedescriptionlanguage(internalmodeDDL).
Thedatamodelgivesthedataframestructureofthedatabase,andthedatainthedatabaseistherealentity,butthesedatamustbeorganizedaccordingtothestructuredescribedbytheframework,andthedatabasecomposedoftheconceptualmodelastheframeworkCalledConceptualDatabase,adatabasecomposedofaframeworkwithanexternalmodeliscalledauserdatabase,andadatabasecomposedofaframeworkwithaninternalmodeliscalledaphysicaldatabase.Amongthethreedatabases,onlythephysicaldatabaseisreal.Existinthecomputer'sexternalmemory,theothertwodatabasesdonotreallyexistinthecomputer,butaremappedfromthephysicaldatabasethroughtwomappings.
Thethreelevelsofthemodelreflectthethreedifferentenvironmentsofthemodelandtheirdifferentrequirements.Theinternalmodelisatthelowestlevel.Itreflectstheactualstorageformofthedatainthephysicalstructureofthecomputer,theconceptualmodelInthemiddlelayer,itreflectsthedesigner'sdatagloballogicrequirements,whiletheoutermodeisintheoutermostlayer,whichreflectstheuser'sdatarequirements.
Thethree-levelmodeofthedatabasesystemisanabstractionofthethreelevelsofdata.Itleavesthespecificphysicalrealizationofthedatatothephysicalmode,sothatusersandglobaldesignersdonotneedtocareaboutthespecificrealizationandphysicalbackgroundofthedatabaseAtthesametime,itestablishestheconnectionandconversionbetweenthethree-levelmodelthroughtwo-levelmapping,sothatalthoughtheconceptualmodelandtheexternalmodeldonothaveaphysicalexistence,theycanalsoobtaintheirexistingentitiesthroughthemapping.Atthesametime,thetwo-levelmappingalsoensuresthedatabaseTheindependenceofthedatainthesystem,thatis,changesinthephysicalorganizationofthedataandchangesinthelogicalconceptlevel,doesnotaffectthechangesintheexternalmodeoftheuser.Itonlyneedstoadjustthemappingmodewithoutchangingtheusermode.
Mappingfromconceptualmodetointernalmode
Thismappinggivesthecorrespondencebetweenthegloballogicalstructureofthedataintheconceptualmodeandthephysicalstoragestructureofthedata.ThismappingisgenerallydefinedbyDBMSimplementation.
Mappingfromexternalmodetoconceptualmode
Conceptmodeisaglobalmodeandexternalmodeisauser'slocalmode.Aconceptualmodecandefinemultipleexternalmodes,andeachTheexternalmodeisabasicviewoftheconceptualmode.Themappingfromtheexternalmodetotheconceptualmodegivesthecorrespondingrelationshipbetweentheexternalmodeandtheconceptualmode,andthismappingisgenerallyimplementedbyaDBMS.