Ten select by mohol vyzerať nejako takto:
select t.*, count(t2.idkomentare) as pocet, max(t2.datum) as poslednydatum
from tabulka as t
left outer join tabulka as t2 on t2.koren=t.idkomentare
where t.koren=0
group by t.idkomentare
order by poslednydatum desc
limit 10