which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by

SET GLOBAL sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));
SET SESSION sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));

SET GLOBAL sql_mode=(SELECT REPLACE(@@sql_mode,'ONLY_FULL_GROUP_BY',''));

mysql> set global sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

mysql> set session sql_mode='STRICT_TRANS_TABLES,NO_ZERO_IN_DATE,NO_ZERO_DATE,ERROR_FOR_DIVISION_BY_ZERO,NO_AUTO_CREATE_USER,NO_ENGINE_SUBSTITUTION';

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')