select
t.Topic,
t.Title,
count(distinct s.starID) as StarCount,
count(distinct m.User) as UserCount,
count(distinct m.messageID) as MessageCount
from
Topics t
left join Messages m ON m.Topic = t.Topic
left join Stars_Given s ON s.Topic = t.Topic
group by
t.Topic,
t.Title
Sql Fiddle
Nebo můžete provést agregaci v dílčích dotazech, což bude pravděpodobně efektivnější, pokud budete mít v tabulkách značné množství dat:
select
t.Topic,
t.Title,
s.StarCount,
m.UserCount,
m.MessageCount
from
Topics t
left join (
select
Topic,
count(distinct User) as UserCount,
count(*) as MessageCount
from Messages
group by Topic
) m ON m.Topic = t.Topic
left join (
select
Topic,
count(*) as StarCount
from Stars_Given
group by Topic
) s ON s.Topic = t.Topic