ydz=# select * from test1;
 id | group_id | flag |   time
----+----------+------+----------
  1 |        1 |    0 | 10:30:00
  2 |        2 |    1 | 11:00:00
  4 |        2 |    0 | 08:30:00
  3 |        1 |    0 | 10:00:00
  5 |        2 |    1 | 08:30:00
  6 |        2 |    1 | 10:50:00
  7 |        1 |    1 | 10:45:00
(7 строк)

ydz=# select * from test1 t inner join (select group_id, max(time) from test1 group by group_id) j on j.group_id = t.group_id where flag = 1 and ('11:00'::time - interval '1 hour')::time < time and j.max = t.time;
 id | group_id | flag |   time   | group_id |   max
----+----------+------+----------+----------+----------
  2 |        2 |    1 | 11:00:00 |        2 | 11:00:00
  7 |        1 |    1 | 10:45:00 |        1 | 10:45:00
(2 строки)

ydz=#