数据库之父对实现关系型数据库管理系统的12条建议

最新推荐文章于 2024-04-23 17:20:14 发布

最新推荐文章于 2024-04-23 17:20:14 发布 · 272 阅读

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本文介绍了数据库之父提出的实现关系型数据库管理系统的12条建议，包括信息规则、保证访问规则等。这些规则为关系型数据库设计提供了指导，虽尚无商业系统完全符合，但仍是设计目标，还将用SQL详细阐释相关概念和数据库结构。

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这是数据库之父对实现关系型数据库管理系统的12条建议:

Codd's12Rules

Dr.E.F.Codd,anIBMresearcher,firstdevelopedtherelationaldatamodelin1970.In1985,Dr.Coddpublishedalistof12rulesthatconciselydefineanidealrelationaldatabase,whichhaveprovidedaguidelineforthedesignofallrelationaldatabasesystemseversince.

Iusetheterm"guideline"because,todate,nocommercialrelationaldatabasesystemfullyconformstoall12rules.Theydorepresenttherelationalideal,though.Forafewyears,scorecardswerekeptthatratedeachcommercialproduct'sconformitytoCodd'srules.Today,therulesarenottalkedaboutasmuchbutremainagoalforrelationaldatabasedesign.

FollowingisalistofCodd's12rules,includinghisoriginalnameforeachruleandasimplifieddescription.Ialsohaveincludedanotewherecertainrulesareproblematictoimplement.Don'tworryifsomeoftheseitemsareconfusingtoyou,aswemovefurtherthroughthisnewsletterserieswewillfillinthedetails.

Rule1:TheInformationRule
Alldatashouldbepresentedtotheuserintableform.Lastweek'snewsletteralreadydiscussedthebasicsofthisrule.

Rule2:GuaranteedAccessRule
Alldatashouldbeaccessiblewithoutambiguity.Thiscanbeaccomplishedthroughacombinationofthetablename,primarykey,andcolumnname.

Rule3:SystematicTreatmentofNullValues
Afieldshouldbeallowedtoremainempty.Thisinvolvesthesupportofanullvalue,whichisdistinctfromanemptystringoranumberwithavalueofzero.Ofcourse,thiscan'tapplytoprimarykeys.Inaddition,mostdatabaseimplementationssupporttheconceptofanun-nullfieldconstraintthatpreventsnullvaluesinaspecifictablecolumn.

Rule4:DynamicOn-LineCatalogBasedontheRelationalModelArelationaldatabasemustprovideaccesstoitsstructurethroughthesametoolsthatareusedtoaccessthedata.Thisisusuallyaccomplishedbystoringthestructuredefinitionwithinspecialsystemtables.

Rule5:ComprehensiveDataSublanguageRule
Thedatabasemustsupportatleastoneclearlydefinedlanguagethatincludesfunctionalityfordatadefinition,datamanipulation,dataintegrity,anddatabasetransactioncontrol.AllcommercialrelationaldatabasesuseformsofthestandardSQL(StructuredQueryLanguage)astheirsupportedcomprehensivelanguage.

Rule6:ViewUpdatingRule
Datacanbepresentedtotheuserindifferentlogicalcombinations,calledviews.Eachviewshouldsupportthesamefullrangeofdatamanipulationthatdirect-accesstoatablehasavailable.Inpractice,providingupdateanddeleteaccesstologicalviewsisdifficultandisnotfullysupportedbyanycurrentdatabase.

Rule7:High-levelInsert,Update,andDelete
Datacanberetrievedfromarelationaldatabaseinsetsconstructedofdatafrommultiplerowsand/ormultipletables.Thisrulestatesthatinsert,update,anddeleteoperationsshouldbesupportedforanyretrievablesetratherthanjustforasinglerowinasingletable.

Rule8:PhysicalDataIndependence
Theuserisisolatedfromthephysicalmethodofstoringandretrievinginformationfromthedatabase.Changescanbemadetotheunderlyingarchitecture(hardware,diskstoragemethods)withoutaffectinghowtheuseraccessesit.

Rule9:LogicalDataIndependence
Howauserviewsdatashouldnotchangewhenthelogicalstructure(tablesstructure)ofthedatabasechanges.Thisruleisparticularlydifficulttosatisfy.Mostdatabasesrelyonstrongtiesbetweentheuserviewofthedataandtheactualstructureoftheunderlyingtables.

Rule10:IntegrityIndependence
Thedatabaselanguage(likeSQL)shouldsupportconstraintsonuserinputthatmaintaindatabaseintegrity.Thisruleisnotfullyimplementedbymostmajorvendors.Ataminimum,alldatabasesdopreservetwoconstraintsthroughSQL.Nocomponentofaprimarykeycanhaveanullvalue.(seerule3)
Ifaforeignkeyisdefinedinonetable,anyvalueinitmustexistasaprimarykeyinanothertable.

Rule11:DistributionIndependence
Ausershouldbetotallyunawareofwhetherornotthedatabaseisdistributed(whetherpartsofthedatabaseexistinmultiplelocations).Avarietyofreasonsmakethisruledifficulttoimplement;Iwillspendtimeaddressingthesereasonswhenwediscussdistributeddatabases.

Rule12:NonsubversionRule
Thereshouldbenowaytomodifythedatabasestructureotherthanthroughthemultiplerowdatabaselanguage(likeSQL).Mostdatabasestodaysupportadministrativetoolsthatallowsomedirectmanipulationofthedatastructure.Overthelifeofthisnewsletter,IwillbeexpandingontheconceptscoveredbyeachofCodd'srules.Iwillusetherelationalquerylanguageofchoice,SQL,toillustratetheseconceptsandexplainrelationaldatabasestructureindetail.

转自: http://www.zhanling.com/zhanling/qiu/blogview.asp?logID=83&cateID=3