MySQL свързва две таблици, намира максимален брой и подрежда по

select daybook.userid, count(*) as count 
    from daybook, rating 
    where daybook.userid = rating.userid
    group by daybook.userid
    order by count desc

Но дори наистина не се нуждаете от таблицата с дневник:

select userid, count(*) as count
   from rating
   group by userid
   order by count desc