MySQL slå samman två bord, hitta maxantal och beställ efter

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

Men du behöver egentligen inte ens dagboksbordet:

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