използвайте IFNULL в laravel

Опитайте така:

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();