Getting the sum of hours between two DATETIME columns

Hi all,

I'm trying to execute the following query to get the sum of hours between two datetime columns based on some cretriea, but the problem that I'm getting null result.

$numberOfHours = DB::table('session')->where('doctor_id', $doctorId) ->whereNotNull('start_time') ->whereBetween('end_time', [$start, $end]) ->select(DB::raw("SUM(TIMESTAMPDIFF(HOUR, 'start_time', 'end_time')) as result"))->get(['result']);

Any suggestions?

Thanks.