I have table e.g
ID START_TIME END_TIME STATUS
1 2011-10-09 23:06:00 2011-10-09 23:13:00 Open
2 2011-10-09 23:13:00 2011-10-09 23:13:00 Assigned (User)
6 2011-10-09 23:13:00 2011-10-10 04:47:00 Waiting for approval (Reassigned)
10 2011-10-10 04:47:00 2011-10-10 04:47:00 Open
11 2011-10-10 04:47:00 2011-10-10 05:00:00 Waiting for approval (Reassigned)
22 2011-10-11 04:40:00 2011-10-11 04:40:00 Resolved
Now, i need the difference between first row (start_time) and last row (end time)