Ich würd's einfach mit 2 Views machen:
1. create view customer_summen as
select cust,sum(qnty) as summe
from tretn
group by cust
2.
select tcust.name,customer_summen.cust,customer_summen.su mme
from tcust inner join customer_summen
on tcust.cust=customer_summen.cust
where summe=
(select max(summe) from customer_summen)
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