Hibernate HQL Query Subselects or Joins
select u.username, count(m), count(p) from User as u
   inner join u.Messages as m
   inner join u.Posts as p
   group by u.id
   order by count(m) desc

select u.username,
       (select count(m.id) from Message m where m.user = u) as messageCount,
       (select count(p.id) from Post p where p.user = u) as postCount
from User u
order by messageCount desc

select u.username, count(distinct m.id), count(distinct p.id) from User u
inner join u.messages m
inner join u.posts p
group by u.username)
order by (count(distinct m.id) desc