WITH t AS
(
SELECT 1 no, 'WORK1' gbn, TO_DATE('2014-05-01', 'yyyy-mm-dd') work_date FROM dual
UNION ALL SELECT 2, 'WORK1', TO_DATE('2014-05-02', 'yyyy-mm-dd') FROM dual
UNION ALL SELECT 3, 'ABC'  , TO_DATE('2014-05-04', 'yyyy-mm-dd') FROM dual
UNION ALL SELECT 4, 'CCC'  , TO_DATE('2014-05-06', 'yyyy-mm-dd') FROM dual
UNION ALL SELECT 5, 'AEC'  , TO_DATE('2014-05-06', 'yyyy-mm-dd') FROM dual
UNION ALL SELECT 6, 'WORK1', TO_DATE('2014-05-07', 'yyyy-mm-dd') FROM dual
UNION ALL SELECT 7, 'CCC'  , TO_DATE('2014-05-08', 'yyyy-mm-dd') FROM dual
)
SELECT MIN(work_date) work_date_from
     , MAX(work_date) work_date_to
     , MAX(work_date)
     - MIN(work_date) work_date_cnt
  FROM (SELECT no, gbn, work_date
             , SUM(DECODE(gbn, 'WORK1', 1)) OVER(ORDER BY no) grp
          FROM (SELECT no, gbn, work_date
                     , LAG(gbn, 1, 'x') OVER(ORDER BY no) lag_gbn
                  FROM t
                )
         WHERE gbn != lag_gbn
           AND 'WORK1' IN (gbn, lag_gbn)
        )
 GROUP BY grp
 ORDER BY MIN(work_date)
;

-- 좀더 간결하게 --
SELECT MIN(work_date) work_date_from
     , MAX(work_date) work_date_to
     , MAX(work_date)
     - MIN(work_date) work_date_cnt
  FROM (SELECT no, gbn, work_date
             , LAG(gbn, 1, 'x') OVER(ORDER BY no) lag_gbn
          FROM t
        )
 WHERE gbn != lag_gbn
   AND 'WORK1' IN (gbn, lag_gbn)
 GROUP BY CEIL(ROWNUM / 2)
 ORDER BY MIN(work_date)
;