--- ORACLE 문장 --
SELECT *
  FROM insa
WHERE DECODE(:as_gubun,'01',strday,'02', endday) BETWEEN '20081201' AND '20081231'

--- 이렇게 해결 --
SELECT *
  FROM insa
WHERE (CASE :as_gubun
        WHEN '01' THEN strday
        WHEN '02' THEN endday
        ELSE :as_sdate END )  BETWEEN :as_sdate  AND :as_edate