WHEREで絞りこまずに
LEFT JOINでANDで絞り込めば良いのね。
結構ハマったけど、初めてじゃない気が・・・
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Shohin LEFT OUTER JOIN ShohinSign
ON ShohinSign.ShohinSign_SignCD = Shohin.Shohin_SignCD
WHERE ShohinSign.ShohinSign_DataKubun = 1
↓↓
Shohin LEFT OUTER JOIN ShohinSign
ON ShohinSign.ShohinSign_SignCD = Shohin.Shohin_SignCD
AND ShohinSign.ShohinSign_DataKubun = 1