数据库中候选码求法.(比较全的哦)（the method of finding candidate codes in database）

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1、数据库中候选码求法.(比较全的哦)（Themethodoffindingcandidatecodesindatabase）First,thespecificstepstosolvethecandidatecodebasicalgorithmThefirststepistofindtheminimumfunctiondependencysetFoftherelationalschemaRThesecondstep,accordingtotheabovedefinition,calculateUL,UR,UB(ULrepresentsthedependencyset;each

2、dependencypropertyrelationappearsonlyintheleftURfunctionrepresentsthedependencyset;eachattributeappearsontherightsideofthedependentrelationonlyinotherfunctionUB=U-UL-UR)Thethirdstep,ifUL=phi,thecalculationofULclosure,ifUL+=U,ULforRcandidatecode,onlytheendofthealgorithm.IfUL+=U,turntostepfourth

3、.IfUL=phi,turntostepfifth.Inthefourthstep,theULiscombinedwiththeattributesinUB,andthecombinationof4isusedtodeterminewhetherthecombinationattributeiscandidatecode.Afterfindingallcandidatecodes,thealgorithmisfinishedInthefifthstep,thecombinationofattributesandattributesinUBisdeterminedbyusingthe

4、abovedefinition4,andallthecandidatecodesarefoundout,andthealgorithmisfinishedInshort:taketheminimumdependencysetandcomputetheULclosure.IftheULclosedbagcontainsfullattributes,thenUListheuniquecandidatecode.Ifnot,theUBattributeissequentiallycombinedtofindwhethertheclosurecontainsfullattributes.W

5、henULisempty,UBisdirectlycombinedtofindclosureTwo,multiattributedependencysetcandidatecodesolutionmethodInput:relationalschemaRanditsfunctionaldependencysetF.Output:allcandidatecodesforR.Specificsteps:1)allattributesofRaredividedintofourcategories:L,R,NandLR,andXistheclassofLandN,andYistheLRcl

6、ass.2)X+,ifX+containsalltheattributesofR,thenXistheonlycandidatecodeofR,turn(5);otherwise,turn(3).3)takeanattributeAinY,find(XA)+,ifitcontainsalltheattributesofR,turn(4);otherwise,swapanattributerepeatedlytothisprocess,untilalltheattributesinYaretried.4)ifallthecandidatecodeshavebeenfound,then

7、turn(5);otherwise,twoandthreeinturninY......SeektheirattributeclosuresuntilallattributesoftheirclosedbagcontainR.5)stop,outputresults.Inshort:apropertyoftheX(XL,N)forclosure,ifalltheattributesare,includingRcode,ortakeaclassLRYattributeA