merge into DB2ADMIN.saldo as a
using (select podrazd_id, debet_id, debet_partner_id, debet_analitika_id, sum(summa) as summa
from old group by podrazd_id, debet_id, debet_partner_id, debet_analitika_id) as b
on a.podrazd_id = b.podrazd_id
and a.subcount_id = b.debet_id
and a.partner_id = b.debet_partner_id
and a.analitika_id = b.debet_analitika_id
when matched then update set summa_debet = summa_debet - b.summa;

update DB2ADMIN.saldo a
set summa_debet=summa_debet-
(
select sum(summa)
from old b
where 
    a.podrazd_id = b.podrazd_id
and a.subcount_id = b.debet_id
and a.partner_id = b.debet_partner_id
and a.analitika_id = b.debet_analitika_id
)
where exists 
(
select sum(summa)
from old b
where 
    a.podrazd_id = b.podrazd_id
and a.subcount_id = b.debet_id
and a.partner_id = b.debet_partner_id
and a.analitika_id = b.debet_analitika_id
);