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Question

私の間違いはどこですか？

select dr.ders_code, dr.year, 
case when dr.muf_sq is not null then 
case when (select muf_sq_id from mufredat_sq m join ders_stud_ref dr on dr.muf_sq = m.muf_sq_id) = dr.muf_sq then dr.muf_sq end 
end 
from ders_stud_ref dr 

Answer 1

最初の間違いは

select muf_sq_id 
       from mufredat_sq m 
       join ders_stud_ref dr on dr.muf_sq = m.muf_sq_id 

である以上、その後1行

を返すことが、私はあなたが

select dr.ders_code, dr.year, 
case when dr.muf_sq is not null then 
case when exists (select muf_sq_id 
      from mufredat_sq m 
      where dr.muf_sq = m.muf_sq_id) then dr.muf_sq end 
end 
from ders_stud_ref dr