Show the account id for each transaction.
select account_id from financial_transactions
For each of them, show the number of transactions.
select account_id, count(*) from financial_transactions group by account_id
Order the ids in descending order of the number of transactions.
select account_id from financial_transactions group by account_id order by count(*) desc
Which id has the most?
select account_id from financial_transactions group by account_id order by count(*) desc limit 1