WITH t(item_no, gb, order_no, order_dt) AS
(
SELECT 'A1234', '입고', 123456, '20161201' UNION ALL
SELECT 'A5678', '입고', 123456, '20161201' UNION ALL
SELECT 'A1234', '반품', 123456, '20161207'
)
-- 1. 셀프조인 --
SELECT a.*
  FROM t a
  LEFT OUTER JOIN t b
    ON b.gb = '반품'
   AND a.item_no = b.item_no
   AND a.order_no = b.order_no
 WHERE a.gb = '입고'
   AND b.gb IS NULL
;
-- 2. 분석함수 --
SELECT item_no, gb, order_no, order_dt
  FROM (SELECT item_no, gb, order_no, order_dt
             , LEAD(gb) OVER(PARTITION BY order_no, item_no
                                 ORDER BY order_dt) x
          FROM t
        ) a
 WHERE gb = '입고'
   AND x IS NULL
;

 예 마농님 감사합니다.

여러모로 도움이 많이 되고 있습니다.  감사합니다.