update s1
set s1.oppvirkulttotaal=s2.result
from snapshot2 as s1
inner join (select kultivar
,sum(opp) as result
from snapshot
group by kultivar
) as s2
on s2.kultivar=s1.kultivar
;
or
update s1
set s1.oppvirkulttotaal=(select sum(opp)
from snapshot as s2
where s2.kultivar=s1.kultivar
)
from snapshot2 as s1
;