//구문을 분석해 보시면 이해가 될듯.. ^&^

update Qna_BoardList set 
New_Cat = b.New_Cat from Qna_BoardList a join Sheet3 b on a.Id = b.Id  WHERE a.Ref = b.
Ref and a.Step = b.Step

 

select a.New_Cat, b.New_Cat from  Qna_BoardList a join Sheet3 b on a.Id = b.Id  WHERE a.R
ef = b.Ref and a.Step = b.Step