WITH tbl_a AS
(
    select '1' gb, '2012-08-13 오전 11:16:08' dt from dual union all
    select '1' gb, '2012-08-13 오전 11:16:09' dt from dual union all
    select '1' gb, '2012-08-13 오전 11:16:11' dt from dual union all
    select '2' gb, '2012-08-13 오전 11:30:50' dt from dual union all
    select '2' gb, '2012-08-13 오전 11:32:19' dt from dual union all
    select '1' gb, '2012-08-13 오전 11:36:58' dt from dual union all
    select '1' gb, '2012-08-13 오전 11:37:29' dt from dual union all
    select '2' gb, '2012-08-13 오전 11:40:24' dt from dual
)
SELECT MIN(gb) gb
     , MIN(dt) sdt
     , MAX(decode(lg, 1, '', dt)) edt
  FROM
  (     
    SELECT gb
         , dt
         , SUM(lg) OVER( ORDER BY dt) grp
         , lg
      FROM
      (    
        SELECT gb
             , dt
             , DECODE( LAG(gb) OVER(ORDER BY dt), gb, 0, 1 ) lg
          FROM tbl_a    
      )
  )      
 GROUP BY grp

SELECT gb
     , MIN(dt) sdt
     , MAX(dt) edt
  FROM (
        SELECT gb, dt
             , ROW_NUMBER() OVER(ORDER BY dt) rn1
             , ROW_NUMBER() OVER(PARTITION BY gb ORDER BY dt) rn2
          FROM tbl_a
        )
 GROUP BY rn1-rn2, gb
 ORDER BY sdt
;