WITH t AS
(
SELECT 'A' gb, 303 seq, '20150817140248' tm FROM dual
UNION ALL SELECT 'A', 316, '20150817140310' FROM dual
UNION ALL SELECT 'A', 432, '20150817153924' FROM dual
UNION ALL SELECT 'A', 426, '20150817153957' FROM dual
UNION ALL SELECT 'A', 423, '20150817154218' FROM dual
UNION ALL SELECT 'A', 101, '20150817154341' FROM dual
UNION ALL SELECT 'A', 112, '20150817154524' FROM dual
UNION ALL SELECT 'A', 117, '20150817154614' FROM dual
UNION ALL SELECT 'A', 118, '20150817154754' FROM dual
UNION ALL SELECT 'A', 117, '20150817163924' FROM dual
UNION ALL SELECT 'A', 230, '20150817164127' FROM dual
UNION ALL SELECT 'A', 262, '20150817164323' FROM dual
UNION ALL SELECT 'A', 229, '20150817164405' FROM dual
UNION ALL SELECT 'A', 229, '20150817171807' FROM dual
UNION ALL SELECT 'A', 262, '20150817172058' FROM dual
UNION ALL SELECT 'A', 230, '20150817172311' FROM dual
UNION ALL SELECT 'A', 112, '20150817172333' FROM dual
UNION ALL SELECT 'A', 101, '20150817172427' FROM dual
UNION ALL SELECT 'A', 401, '20150817172531' FROM dual
UNION ALL SELECT 'A', 423, '20150817172559' FROM dual
UNION ALL SELECT 'A', 424, '20150817172749' FROM dual
UNION ALL SELECT 'A', 426, '20150817172833' FROM dual
UNION ALL SELECT 'A', 432, '20150817172903' FROM dual
)
SELECT gb
     , grp
     , LISTAGG(seq, ',') WITHIN GROUP(ORDER BY tm) x
  FROM (SELECT gb, seq, tm
             , SUM(flag) OVER(PARTITION BY gb ORDER BY tm) grp
          FROM (SELECT gb, seq, tm
                     , CASE WHEN LAG(tm) OVER(PARTITION BY gb ORDER BY tm)
                               > tm - 1/24/60 * 30
                            THEN 0 ELSE 1 END flag
                  FROM (SELECT gb, seq
                             , TO_DATE(tm, 'yyyymmddhh24miss') tm
                          FROM t
                        )
                )
        )
 GROUP BY gb, grp
 ORDER BY gb, grp
;