TransformationRule(逻辑实现成关系表的规则）

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wordunderlinedisprimarykey
Rule1.EachentityinanE-Rdiagramismappedtoasingletableinarelationaldatabase;thetableisnameaftertheentity.Thecolumnsofthetablerepresentallthesingle-valuedsimpleattributesthatareattachedtotheentity(possiblythroughacompositeattribute,althoughacompositeattributeitsselfdoesnotbecomeacolumnofthetable)。

Rule2:GivenanentityEwithprimaryidentifierp,amulti-valuedattributeaattachedtoEinanE-Rdiagramismappedtoatableofitsown;thetableisnamedafterthepluralmulti-valuedattribute.Thecolumnsofthisnewtablearenamedafterpanda,androwsofthetablecorrespondto(p,a)valuepairs,representingallpairingsofattributevaluesofaassociatedwithentityoccurrencesinE.Theprimarykeyattributeforthistableisthesetofcolumnsinpanda.
EG:(herep==eid,a==hobby)
employes(eid,staddress,city,states,zipcodes)(197,7Beacon,Boston,MA,02102)(221,19BrightonSt,Boston,MA,02103)
hobbies(eid,hobby)(197,chess)(197,painting)(221,reading)

Rules3:N-NRelationships.WhentwoentitiesEandFtakepartinamany-to-manybinaryrelationshipR,therelationshipismappedtoarepresentativetableTintherelatedrelationaldatabasedesign.ThetablecontainscolumnsforallarrtibutesintheprimarykeysofbothtablestransformedfromentitiesEandF,andthissetofcolumnsformstheprimarykeyforthetableT.
EG:(Employees－(1,N)－works_on－(0,N)－Projects        －－－N-N(many-to-many) )
employees(eid,straddr,city,state,zipcode) －－woks_on(eid,prid,percent)－－projects(prid,proj_name,due_date)

Rules4:N-1Relationships.WhentwoentitiesEandFtakepartinamany-to-onebinaryrelationshipR,therlationshipwillnotbemappedtoatableofitsowninarelationaldatabasedesign.Instead,ifweassumethattheentityFhasmax-card(F,R)=1andthusrepresentsthe“many“sideoftherelationship,therelationaltableTtransformedfromtheentityFshouldincludecolumsconstituting theprimarykeyforthetabletransformedfromtheentityE;thisisknownasaforeignkeyinT.
EG:instructors(insid,lname,office_no,ext) －－cours_sections(secid,insid,course,room,preriod).

Rules5:1-1Relationships,optionalParticipation.GiventwoentitiesEandFthattakepartinaone-to-onebinaryrelationshipR,whereparticipationisoptionaloneitherside,wewishtotranslatethissituationintoarelaionaldesign --Useforeignkeyoneitherorbothsides.

Rules6:1-1Relationships,MandatoryParticipationonBothSides.Inthecaseofaone-to-onerelationshipwithmandatoryparticpationonbothsides,itis mostappropriateto combinethetablesforthetwoentitiesintoone,andinthiswayavoidanyforeignkeys. 

 

 

 

 

 

 

 

 

 

 
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