-- 1. OR --
SELECT *
  FROM tbl
 WHERE (col_yn = 'Y' AND col_val <= 100)
    OR (col_yn = 'N' AND col_val  = 100)
;
-- 2. 굳이 CASE 를 사용하고자 한다면 다음과 같이 억지스럽게 --
SELECT *
  FROM tbl
 WHERE col_val <= 100
   AND col_val >= DECODE(col_yn, 'Y', 0, 'N', 100)
;

 
-- 깔끔치 않네요 ㅡㅡ
WITH T AS (
SELECT 1 SEQ , 'Y' COL_YN , 100 COL_VAL FROM DUAL UNION ALL
SELECT 2 , 'N' , 200 FROM DUAL UNION ALL
SELECT 3 , 'Y' , 50  FROM DUAL UNION ALL
SELECT 4 , 'N' , 100 FROM DUAL 
)

SELECT T.SEQ , T.COL_YN , T.COL_VAL FROM T 
 WHERE SIGN(T.COL_VAL - DECODE(T.COl_YN , 'Y' , 101 , 'N' , 100 )) = DECODE(T.COl_YN , 'Y' , -1 , 'N' , 0 )