tila
Definition:Alsocalledlogicalmode,itisthedescriptionofthelogicalstructureandcharacteristicsofalldatainthedatabase,andisthecommondataviewforallusers.
Ymmärtäminen:
① Tietokanta on vain yksitila;
②Se on tietopohjainen näkymä loogiselle tasolle;
③TietokantaTila perustuu tiettyyn tietomalliin;
④Whendefiningthemode,itisnotonlynecessarytodefinethelogicalstructureofthedata(suchaswhichdataitemsconstitutethedatarecord,thename,type,valuerangeofthedataitem,etc.),Anddefinethesecurityandintegrityrequirementsrelatedtothedata,anddefinetherelationshipbetweenthesedata.
Ulkoinen skeema
Definition:alsoknownasSubschemaorusermode,whichcanbeseenandusedbydatabaseusers(includingapplicationprogrammersandendusers)Thedescriptionofthelogicalstructureandcharacteristicsofthelocaldataisthedataviewofthedatabaseuserandthelogicalrepresentationofthedatarelatedtoacertainapplication.
Ymmärtäminen:
①Tietokannassa voi olla useita ulkoisia tiloja;
②Ulkoinen tila on käyttäjänäkymä;
③Ulkoinen tilaSe on tehokas keino varmistaa tietoturva.
Sisäinen skeema
Definition:alsoknownasStorageSchema,itisthedescriptionofthephysicalstructureandstorageofdata,andtherepresentationofdatainthedatabase(Forexample,isthestoragemethodofrecordsstoredsequentially,storedinaB-treestructure,orstoredinahashmethod;howtheindexisorganized;whetherthedataiscompressedandstoredandwhetheritisencrypted;whataretherequirementsforthestoragerecordstructureofthedata).
Ymmärtäminen:
①Tietokannassa on vain yksi sisäinen skeema;
②Atablemayconsistofmultiplefiles,suchasdatafilesandindexfiles.
Itisamethodforthedatabasemanagementsystem(DBMS)toeffectivelyorganizeandmanagethedatainthedatabase
Sen tarkoitus:
①Inordertoreducedataredundancy,Realizedatasharing;
②Ei parantaapääsytehokkuutta ja suorituskykyä.
Kaavion tulkinta (käsitteellinen skeema)
ConceptualSchemaisthedescriptionoftheglobaldatalogicalstructureinthedatabasesystem,andisthecommondataviewofallusers(applications).ThisdescriptionisanabstractionThedescriptionof,itdoesnotinvolvespecifichardwareenvironmentandplatform,andhasnothingtodowithspecificsoftwareenvironment.
Theconceptualmodelmainlydescribestheconceptualrecordtypeofdataandthedataandtherelationshipbetweenthem.Italsoincludessomesemanticconstraintsbetweenthedata.ItsdescriptioncanbedefinedbytheDDLlanguageintheDBMS.
Ulkoinen skeema
Ulkoinen skeemaisalsocalledSubschemaorUser'sschema.Itistheuser’sdataview,thatis,theuser’sApartoftheseenpatternisderivedfromtheconceptualpattern.Theconceptualpatterngivesaglobaldatadescriptionofthesystemandtheouterpatterngivesalocaldescriptionofeachuser.Aconceptualmodecanhaveseveralexternalmodes,andeachuseronlycaresaboutthemoderelatedtoit.Thiscanshieldalargeamountofirrelevantinformationandisbeneficialtodataprotection,soitisextremelybeneficialtousers.IngeneralDBMS,therearerelatedexternalmodedescriptionlanguages(externalmodeDDL).
Sisäinen skeema
Sisäinen skeemaisalsocalledPhysicalSchema,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.
Kartoitus käsitteellisestä tilastasisäiseen tilaan
Thismappinggivesthecorrespondencebetweenthegloballogicalstructureofthedataintheconceptualmodeandthephysicalstoragestructureofthedata.ThismappingisgenerallydefinedbyDBMSimplementation.
Kartoitus ulkoisesta tilasta käsitteelliseen tilaan
Conceptmodeisaglobalmodeandexternalmodeisauser'slocalmode.Aconceptualmodecandefinemultipleexternalmodes,andeachTheexternalmodeisabasicviewoftheconceptualmode.Themappingfromtheexternalmodetotheconceptualmodegivesthecorrespondingrelationshipbetweentheexternalmodeandtheconceptualmode,andthismappingisgenerallyimplementedbyaDBMS.