WITH t(a, b, c) AS
(
SELECT 'a101', 'a01', 'a01' FROM dual
UNION ALL SELECT 'a102', 'a01', 'a02' FROM dual
UNION ALL SELECT  null , 'a02', 'a03' FROM dual
UNION ALL SELECT  null , 'a03', 'a04' FROM dual
UNION ALL SELECT 'b205', 'a04', 'a05' FROM dual
UNION ALL SELECT 'b208', 'a06', 'a06' FROM dual
UNION ALL SELECT  null , 'a06', 'a07' FROM dual
UNION ALL SELECT  null , 'a06', 'a08' FROM dual
UNION ALL SELECT  null , 'a09', 'a09' FROM dual
UNION ALL SELECT 'c304', 'a10', 'a10' FROM dual
UNION ALL SELECT 'c304', 'a10', 'a10' FROM dual
UNION ALL SELECT 'c304', 'a10', 'a10' FROM dual
UNION ALL SELECT  null , 'a10', 'a10' FROM dual
)
SELECT a, b, c
     , CASE WHEN NVL(rn - r_a, 999) <= NVL(r_d - rn, 999)
            THEN d_a
            ELSE d_d
        END d
  FROM (
        SELECT a, b, c
             , rn
             , LAST_VALUE(r IGNORE NULLS) OVER(ORDER BY rn ASC ) r_a
             , LAST_VALUE(r IGNORE NULLS) OVER(ORDER BY rn DESC) r_d
             , LAST_VALUE(a IGNORE NULLS) OVER(ORDER BY rn ASC ) d_a
             , LAST_VALUE(a IGNORE NULLS) OVER(ORDER BY rn DESC) d_d
          FROM (
                SELECT a, b, c
                     , ROW_NUMBER() OVER(ORDER BY b, c) rn
                     , NVL2(a, ROW_NUMBER() OVER(ORDER BY b, c), null) r
                  FROM t
                )
        )
 ORDER BY rn
;

너무 도움 많이 되었습니다. 감사합니다. ^^

이렇게 간단히 풀어주시다니, 대단하십니다.