Starting from the transactions details, you can join the customers table twice (once for account_from
and once for account_to
) and filter out those that have the same customer_id
. The final step is aggregation:
select sum(td.euro_amount) sum_euro_amount
from transaction_details td
inner join customer c1 on c1.account = td.account_from
inner join customer c2 on c2.account = td.account_to
where c1.customer_id <> c2.customer_id
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