SELECT SEC_TO_TIME(SUM(TIME_TO_SEC(TIMEDIFF(date_out,date_in)))) as total FROM mytable
Also you can find this using php code which is Find total time using mysql query when you have two datetime fields.
function sum_the_time($time1, $time2) {
$times = array($time1, $time2);
$seconds = 0;
foreach ($times as $time)
{
list($hour,$minute,$second) = explode(':', $time);
$seconds += $hour*3600;
$seconds += $minute*60;
$seconds += $second;
}
$hours = floor($seconds/3600);
$seconds -= $hours*3600;
$minutes = floor($seconds/60);
$seconds -= $minutes*60;
// return "{$hours}:{$minutes}:{$seconds}";
return sprintf('%02d:%02d:%02d', $hours, $minutes, $seconds);
}
function diff_the_time($time1, $time2) {
$seconds=strtotime($time1)-strtotime($time2);
$hours = floor($seconds/3600);
$seconds -= $hours*3600;
$minutes = floor($seconds/60);
$seconds -= $minutes*60;
// return "{$hours}:{$minutes}:{$seconds}";
return sprintf('%02d:%02d:%02d', $hours, $minutes, $seconds);
}