1. set strict to false in config/databtase.php 
2. ->select([
        'exam_registrations.id',
        'exam_registrations.exam_id',
        'exam_registrations.user_id',
        DB::raw('sum(submissions.marks) as totalMarksGained')
     ])
     ->groupBy('exam_registrations.user_id')
     ->orderBy('totalMarksGained', 'DESC')