-- 1. UNION ALL
SELECT '' a, '' b, '' c
     , ROUND(SUM(DECODE(a, '가', d)) / SUM(DECODE(a, '나', d)) * 100, 2) d
  FROM t
 WHERE a IN ('가', '나')
 UNION ALL
SELECT *
  FROM (SELECT a, b, c, d FROM t ORDER BY a, b, c)
 UNION ALL
SELECT '' a, '' b, '' c
     , SUM(d) d
  FROM t
 WHERE a IN ('나', '다')
;

-- 2. ROLLUP 에서 상수값 1 이용
SELECT a, b, c
     , NVL(d, DECODE(GROUPING(1)
     , 0, ROUND(SUM(DECODE(a, '가', d)) / SUM(DECODE(a, '나', d)) * 100, 2)
     , 1, SUM(CASE WHEN a IN ('나', '다') THEN d END)
     ) ) d
  FROM t
 GROUP BY ROLLUP(1, (a, b, c, d))
 ORDER BY GROUPING(1), a NULLS FIRST, b, c
;

-- 3. 분석함수를 이용해 값을 구한 뒤 ROLLUP 이용
SELECT a, b, c
     , COALESCE(d, e, f) d
  FROM (SELECT a, b, c, d
             , ROUND( SUM(DECODE(a, '가', d)) OVER()
                    / SUM(DECODE(a, '나', d)) OVER()
                    * 100, 2) e
             , SUM(CASE WHEN a IN ('나', '다') THEN d END) OVER() f
          FROM t
        )
 GROUP BY f, ROLLUP(e, (a, b, c, d))
 ORDER BY GROUPING(e), a NULLS FIRST, b, c
;

-- 4. Model 절 이용
SELECT a, b, c, d
  FROM t
 MODEL
 DIMENSION BY (ROW_NUMBER() OVER(ORDER BY a, b, c) rn, a)
 MEASURES (b, c, d)
 RULES
 ( d[ 0, ''] = ROUND(SUM(d)[ANY, '가'] / SUM(d)[ANY, '나'] * 100, 2)
 , d[99, ''] = SUM(d)[ANY, a IN ('나', '다')]
 )
 ORDER BY rn
;

역시 마농님

감사합니다.ㅠ