한번만 읽고 하는 방법은 딱히 없어보입니다만..

exists 문을 사용하여 부분범위처리로 하면 그나마 응답속도는 빨라보이는데요..

WITH  A AS (
            SELECT 1 AS A, NULL B FROM DUAL UNION
            SELECT 2 AS A, NULL B FROM DUAL UNION
            SELECT 3 AS A, 1    B FROM DUAL UNION
            SELECT 4 AS A, NULL B FROM DUAL UNION
            SELECT 5 AS A, 2    B FROM DUAL UNION
            SELECT 6 AS A, NULL B FROM DUAL UNION
            SELECT 7 AS A, NULL B FROM DUAL UNION
            SELECT 8 AS A, 4    B FROM DUAL UNION
            SELECT 9 AS A, NULL B FROM DUAL
            )

SELECT *
  FROM A
 WHERE A NOT IN ( SELECT B FROM A WHERE B IS NOT NULL)
   AND CASE WHEN A IS NOT NULL AND B IS NOT NULL THEN 'Y'
            ELSE 'N'
       END = 'N'

장마철님 글처럼 쿼리하시면 데이터가 많을때 상당히 느려지는 상황이 발생할수 있습니다.

WITH  tableA AS (

            SELECT 1 AS A, NULL B FROM DUAL UNION
            SELECT 2 AS A, NULL B FROM DUAL UNION
            SELECT 3 AS A, 1    B FROM DUAL UNION
            SELECT 4 AS A, NULL B FROM DUAL UNION
            SELECT 5 AS A, 2    B FROM DUAL UNION
            SELECT 6 AS A, NULL B FROM DUAL UNION
            SELECT 7 AS A, NULL B FROM DUAL UNION
            SELECT 8 AS A, 4    B FROM DUAL UNION
            SELECT 9 AS A, NULL B FROM DUAL
            )

SELECT *
  FROM tableA a
 WHERE B IS NULL

      AND NOT EXISTS (SELECT 1 FROM tableA b WHERE a.A=b.B AND ROWNUM=1)

이렇게 하심 반응은 좀 빨라지겠네요

WITH t AS
(
SELECT 1 a, Null b FROM dual
UNION ALL SELECT 2, Null FROM dual
UNION ALL SELECT 3,    1 FROM dual
UNION ALL SELECT 4, Null FROM dual
UNION ALL SELECT 5,    2 FROM dual
UNION ALL SELECT 6, Null FROM dual
UNION ALL SELECT 7, Null FROM dual
UNION ALL SELECT 8,    4 FROM dual
UNION ALL SELECT 9, Null FROM dual
)
SELECT NVL(b, a) a
  FROM t
 GROUP BY NVL(b, a)
HAVING COUNT(*) = 1
;

오랫동안 고민하다 역시 이곳에서 답을 찾았습니다.

답변주신분들께 감사 인사 드립니다.

감사합니다.