Essayez comme ceci :
DB::Raw('IFNULL( `downloads`.`is_download` , 0 )');
$subjects = $app->db->table('subjects')->LeftJoin('downloads', 'subjects.subjectID', '=', 'downloads.subject_id')
->where('universityID', $currentUser->universityID)->where('semesterID', $currentUser->semesterID)->where('courseID', $currentUser->courseID)
->select('subjects.subjectID', 'subjects.subjectName', 'subjects.price', DB::Raw('IFNULL( `downloads`.`is_download` , 0 )') )
->orderBy('subjectID')
->get();