MSSQL教程之Building a T-SQL Loop
对于insert和delete，event中包含了插入/删除的记录的所有字段的值(太爽了。。)November5,2003T-SQLProgrammingPart2-BuildingaT-SQLLoop
ByGregoryA.Larsen
ThisisthesecondarticleinmyT-SQLprogrammingseries.ThisarticlewilldiscussbuildingaprogramloopusingT-SQL.Inadditiontotalkingaboutbuildingaloop,Iwillalsodiscusswaysofcontrollingtheloopprocessing,anddifferentmethodstobreakoutofaloop.
Aprogrammingloopisachunkofcodethatisexecutedoverandoveragain.Intheloopsomelogicisexecutedrepeatedlyinaniterativefashionuntilsomeconditionismetthatallowsthecodetobreakoutoftheloop.Oneexampleofwhereyoumightusealoopwouldbetoprocessthroughasetofrecordsonerecordatatime.Anotherexamplemightbewhereyouneedtogeneratesometestdataandaloopwouldallowyoutoinsertarecordintoyourtestdatatablewithslightlydifferentcolumnvalues,eachtimetheloopisexecuted.InthisarticleIwilldiscusstheWHILE,BREAK,CONTINUE,andGOTOstatements.
WHILEStatement
InT-SQLtheWHILEstatementisthemostcommonlyusedwaytoexecutealoop.HereisthebasicsyntaxforaWHILEloop:
WHILE<Booleanexpression><codeblock>
Wherea<Booleanexpression>isanyexpressionthatequatestoatrueorfalseanswer,andthe<codeblock>isthedesirecodetobeexecutedwhilethe<Booleanexpression>istrue.Letsgothrougharealsimpleexample.
InthisexampleIwillincrementacounterfrom1to10anddisplaythecountereachtimethroughtheWHILEloop.
declare@counterintset@counter=0while@counter<10beginset@counter=@counter+1printThecounteris+cast(@counteraschar)end
HerethecodeexecutestheWHILEstatementaslongasthe@counterintegervariableislessthan10,thisistheBooleanexpressionoftheWHILEloop.The@countervariablestartsoutatzero,andeachtimethroughtheWHILEloopitisincrementedby1.ThePRINTstatementdisplaysthevalueinthe@countervariableeachtimethroughtheWHILEloop.Theoutputfromthissamplelookslikethis:
Thecounteris1Thecounteris2Thecounteris3Thecounteris4Thecounteris5Thecounteris6Thecounteris7Thecounteris8Thecounteris9Thecounteris10
Asyoucansee,oncethe@countervariablereaches10theBooleanexpressionthatiscontrollingtheWHILEloopisnolongertrue,sothecodewithinthewhileloopisnolongerexecuted.
Notonlycanyouhaveasinglewhileloop,butyoucanhaveWHILEloopsinsideWHILEloops.OrcommonlyknowasnestingofWHILEloops.Therearelotsofdifferentuseswherenestingisvaluable.IcommonlyusenestingofWHILEloopstogeneratetestdata.MynextexamplewillusetheWHILElooptogeneratetestrecordsforaPARTtable.AgivenPARTrecordisuniquelyidentifiedbyaPart_Id,andaCategory_Id.ForeachPart_IdtherearethreedifferentCategory_Ids.Hereismyexamplethatgenerates6uniquerecordsformyPARTtableusinganestedWHILEloop.
declare@Part_Idintdeclare@Category_Idintdeclare@Descvarchar(50)createtablePART(Part_Idint,Category_Idint,Descriptionvarchar(50))set@Part_Id=0set@Category_Id=0while@Part_Id<2beginset@Part_Id=@Part_Id+1while@Category_Id<3beginset@Category_Id=@Category_Id+1set@Desc=Part_Idis+cast(@Part_Idaschar(1))+Category_Id+cast(@Category_Idaschar(1))insertintoPARTvalues(@Part_Id,@Category_Id,@Desc)endset@Category_Id=0endselect*fromPARTdroptablePART
HereistheoutputfromtheSELECTstatementatthebottomofthisnestedWHILEloopexample.
Part_IdCategory_IdDescription---------------------------------------------------------------11Part_Idis1Category_Id112Part_Idis1Category_Id213Part_Idis1Category_Id321Part_Idis2Category_Id122Part_Idis2Category_Id223Part_Idis2Category_Id3
Asyoucansee,byusinganestedWHILEloopeachcombinationofPart_IdandCategory_Idisunique.ThecodewithinthefirstWHILEloopcontrolledtheincrementingofthePart_Id,whereasthesecondWHILEloopsettheCategory_Idtoadifferentvalueeachtimethroughtheloop.Thecodewithinthefirstwhileloopwasexecutedonlytwice,butthecodeinsidethesecondWHILEloopwasexecuted6times.Thusgivingme6samplePARTrecords.
BREAKandCONTINUEStatements
Nowsometimesyouwanttobuildaloopthatwillprocessthroughlogicallytotheendmostofthetime,butnotallthetime.Inotherwords,youmaywanttobreakoutoftheloopifsomeparticularconditionarises.Alsoinadditiontobreakingoutoftheloop,youmaynotwanttoprocessallthecodeintheloopbeforegoingbacktothetopoftheloopandstartingthroughthenextiterationoftheloop.ForthesekindsofprogrammingrequirementsSQLServerprovidestheBREAKandCONTINUEstatements.
TheBREAKstatementexitsoutoftheinnermostWHILEloop,andproceedstothestatementfollowingtheENDstatementthatisassociatedwiththeloopinwhichtheBREAKstatementisexecuted.TheCONTINUEstatementskipsexecutingtherestofthestatementsbetweentheCONTINUEstatementandtheENDstatementofthecurrentloopandstartsexecutingatthefirstlinefollowingtheBEGINstatementofthecurrentWHILEloop.LetsgothoughacoupleofBREAKandCONTINUEexamples.
FortheBREAKstatementImgoingtomodifymylastexamplethatgeneratedPARTtablerecords.ThistimeImgoingtoBREAKoutoftheinnerWHILEloopwhenCategory_IDis2andPART_IDis1.HereismycodefortheBREAKstatement.
declare@Part_Idintdeclare@Category_Idintdeclare@Descvarchar(50)createtablePART(Part_Idint,Category_Idint,Descriptionvarchar(50))set@Part_Id=0set@Category_Id=0while@Part_Id<2beginset@Part_Id=@Part_Id+1while@Category_Id<3beginset@Category_Id=@Category_Id+1If@Category_ID=2and@Part_ID=1Breakset@Desc=Part_Idis+cast(@Part_Idaschar(1))+Category_Id+cast(@Category_Idaschar(1))insertintoPARTvalues(@Part_Id,@Category_Id,@Desc)endset@Category_Id=0endselect*fromPARTdroptablePART
HereistheoutputforthiscodethatcontainsaBREAKstatementinsidetheinnerWHILEloop.
Part_IdCategory_IdDescription---------------------------------------------------------------11Part_Idis1Category_Id121Part_Idis2Category_Id122Part_Idis2Category_Id223Part_Idis2Category_Id3
FromthisoutputyoucanseethatnorecordswereinsertedforPart_Id=1andCategory_Id=2or3,whereastherearerecordsforPart_Id=2withallvaluesfortheCategory_Idcolumn.ThisisbecausetheIFstatementintheinnerloopforcedtheBREAKstatementtoexittheinnerloop.SincetherewererecordsgenerateforPart_Id=2,showsthattheBREAKstatementonlyexitedtheinnerloopandnottheouterloop.
NowjusttostaywiththesameexampleIvebeenusing,letsreplacetheBREAKstatementinthecodeabovewithaCONTINUEstatement.HereisthecodefordemonstratingtheCONTINUEstatement.
declare@Part_Idintdeclare@Category_Idintdeclare@Descvarchar(50)createtablePART(Part_Idint,Category_Idint,Descriptionvarchar(50))set@Part_Id=0set@Category_Id=0while@Part_Id<2beginset@Part_Id=@Part_Id+1while@Category_Id<3beginset@Category_Id=@Category_Id+1If@Category_ID=2and@Part_ID=1Continueset@Desc=Part_Idis+cast(@Part_Idaschar(1))+Category_Id+cast(@Category_Idaschar(1))insertintoPARTvalues(@Part_Id,@Category_Id,@Desc)endset@Category_Id=0endselect*fromPARTdroptablePART
WhenyouusetheCONTINUEstatementyougetthefollowingoutput.
---------------------------------------------------------------11Part_Idis1Category_Id113Part_Idis1Category_Id321Part_Idis2Category_Id122Part_Idis2Category_Id223Part_Idis2Category_Id3
Asyoucansee,whenIusetheCONTINUEstatementonlytherecordwithCategory_Id=2andPart_Id=1ismissing.ThisisbecausetheCONTINUEstatementdoesnotbreakoutoftheinnerWHILEloopbutonlygoesbacktothetopoftheWHILEloopwithoutinsertingtherecord.ThishappensonlywhenCategory_Idis2andPart_Idisequalto1.WhenPart_Id=1andCategory_Id=3theinsertstatementisstillexecuted.
GOTOStatement
TheBREAKstatementwillonlyexityoufromthecurrentlyprocessingWHILEloop,itwillnotbreakoutofallWHILEloops.However,occasionallythisisthekindoffunctionalityyourT-SQLscriptneeds.TohaveyourcodebreakoutofallWHILEloops,nomatterhowmanynestedWHILEstatementsyouhave,youwillneedtousetheGOTOstatement.NowIknowmostprogrammerscringeatthethoughtofusingtheGOTOstatement,butinthiscaseIfeeltheGOTOisanexceptablepractice.UsingmysameexampleIwillusetheGOTOtobreakoutofbothWHILEloops,whenthePART_Id=1andtheCategory_ID=3.
declare@Part_Idintdeclare@Category_Idintdeclare@Descvarchar(50)createtablePART(Part_Idint,Category_Idint,Descriptionvarchar(50))set@Part_Id=0set@Category_Id=0while@Part_Id<2beginset@Part_Id=@Part_Id+1while@Category_Id<3beginset@Category_Id=@Category_Id+1If@Category_ID=3and@Part_ID=1GOTOBREAK_OUTset@Desc=Part_Idis+cast(@Part_Idaschar(1))+Category_Id+cast(@Category_Idaschar(1))insertintoPARTvalues(@Part_Id,@Category_Id,@Desc)endset@Category_Id=0endBREAK_OUT:select*fromPARTdroptablePART
HereistheoutputfromthisGOTOcode:
Part_IdCategory_IdDescription---------------------------------------------------------------11Part_Idis1Category_Id112Part_Idis1Category_Id2
HeretheGOTOlogicstoppedtheinsertionofrecordsintothePARTtablewhen@Category_ID=3and@Part_Id=1.Thisisdonebyexecutingthe"GOTOBREAKOUT"statement.NotethatwhenthisGOTOstatementwasexecuteditbranchedtothelabel"BREAKOUT:"whichcanbefoundfollowingtheENDstatementforthefirst,outermostWHILEstatement.
Conclusion
HopefullynowyouhaveabetterideaofhowtocodeaT-SQLWHILEloop.IveexplainedhowtocontroltheWHILEloop,breakoutofaloopbyusingtheBREAKstatement,usetheCONTINUEstatementtoskipsomeofthecodeinthewhileloop,and/orbreakoutofallWHILEloopsusingtheGOTOstatement.ThetechniquesIvedescribedshouldgiveyouthebasisforbuildingallyourWHILEstatementsfromasingleWHILElooptoacomplexsetofnestedWHILEloops.Mynextarticleinthisserieswilldiscusshowtoprocessthroughasetofrecords.
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