やりたいこと：@P_1が'-'以外の値を取る時は@P_1を採用、それ以外ならwhere句からSpeciesの条件を外す DECLARE @P1 NVARCHAR(64) SET @P1 = '-' SELECT * FROM Iris WHERE ISNULL(Species,'N') = CASE WHEN @P1 <> '-' THEN @P1 ELSE ISNULL(Species,'N') END

P1:category1 P2:category2 SELECT DISTINCT T1.[INDEX], T1.P1, T1.P2, T1.SepalLength, T2.P25_SL, T2.P75_SL, T2.[P75-P25_SL], T2.QL_SL, T2.QH_SL, T1.SepalWidth, T2.P25_SW, T2.P75_SW, T2.[P75-P25_SW], T2.QL_SW, T2.QH_SL, CASE WHEN (T1.SepalLen…