Это запрос
SELECT book_id, AVG(rating) as average
FROM bookReviews
GROUP BY book_id
HAVING count(*) > 5;
Это построитель запросов;
return DB::table('bookReviews')
->select('book_id', DB::raw('AVG(rating) as average'))
->groupBy('book_id')
->having(DB::raw('count(*)'), '>', 5)
->get();