WITH T(NM, SZ ,분류 ,QTY ) AS (
SELECT '메뚜기','2013001','곤충','10' FROM DUAL UNION ALL
SELECT '메뚜기','2013002','곤충','20' FROM DUAL UNION ALL
SELECT '잠자리','2013001','곤충','30' FROM DUAL UNION ALL
SELECT '잠자리','2013002','곤충','10' FROM DUAL UNION ALL
SELECT '개구리','2013001','곤충','20' FROM DUAL UNION ALL
SELECT '사마귀','2013001','곤충','20' FROM DUAL UNION ALL
SELECT '거 미','2013001','곤충','30' FROM DUAL 
)

SELECT NM   "NAME"
   ,MAX(SZ) "SIZE"
   ,분류
   ,MAX(QTY) KEEP(DENSE_RANK LAST ORDER BY SZ) "QTY"
 FROM T
 GROUP BY NM,분류
 

-- 컬럼이 몇개 안될때는 Group By 와 Keep 을 사용하시면 되구요.
-- 컬럼이 많을 때는 Row_Number 를 이용하세요.
SELECT name, size, 분류, qty
  FROM (SELECT name, size, 분류, qty
             , ROW_NUMBER() OVER(PARTITION BY name ORDER BY size DESC) rn
          FROM t
        )
 WHERE rn = 1
;