datediff(hour, o.vytvorena, o.potvrzena) as hodin
from zbozi z
join polozka p on p.zid = z.zid
join objednavka o on o.oid = p.oid
where o.potvrzena IS NOT NULL
group by z.zid, o.vytvorena, o.potvrzena, z.nazev, o.oid
having datediff(hour, o.vytvorena, o.potvrzena) = (

select MIN(hodin.hodin2) from (
	select datediff(hour, ob.vytvorena, ob.potvrzena) as hodin2
	from zbozi zb
	join polozka po on po.zid=zb.zid
	join objednavka ob on ob.oid = po.oid
	where ob.potvrzena IS NOT NULL and zb.zid = z.zid
) hodin
)