Why is there a performance difference between two similar queries where one uses union and the other uses union all?
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CREATE TABLE [dbo].[HPMS_CompetencyTypes](CompetencyType varchar(50) ) go create trigger hpms_create_Insert on HPMS_CompetencyTypes for insert as if Exists ( select * from [HPMS_CompetencyTypes] where CompetencyType=(select * from [HPMS_CompetencyTypes])) begin Rollback tran Raiserror ('duplicate value',12,6) go insert HPMS_CompetencyTypes (CompetencyType) values ('new') I'm new to trigger can any one tell me where is the issue. Please.
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