DECLARE @SerialNo CHAR(5)
SELECT * FROM TblA WHERE SNo=@SerialNo AND ItemNo=
isnull((SELECT ItemNo FROM TblB WHERE ...),0)
SELECT * FROM TblA WHERE SNo=@SerialNo AND ItemNo=
isnull((SELECT ItemNo FROM TblB WHERE ...),0)
DECLARE @SerialNo CHAR(5)
SELECT * FROM TblA WHERE SNo=@SerialNo AND ItemNo=IsNull(
(SELECT ItemNo FROM TblB WHERE ...),0)