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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