WITH T(COL1) AS
(
SELECT 150 FROM DUAL UNION ALL
SELECT 100 FROM DUAL UNION ALL
SELECT 200 FROM DUAL
)
SELECT DECODE(COUNT(DISTINCT COL1),1,'Y','N') OK_OR_NO
  FROM T

해당 컬럼 특정  ROW가 NULL일 경우는요? 제외하나요. 하나의 값으로 보나요?

WITH T AS (
SELECT 150 AS RST_QTY FROM DUAL UNION ALL
SELECT 100 AS RST_QTY FROM DUAL UNION ALL
SELECT 200 AS RST_QTY FROM DUAL
)

SELECT CASE WHEN MAX(RST_QTY) = MIN(RST_QTY) THEN 'OK' ELSE 'NO' END 
  FROM T

감사합니다! MIN MAX를 활용할수도 있군요 캬..