SELECT transaction_type FROM Financial_transactions GROUP BY transaction_type ORDER BY sum(transaction_amount) DESC LIMIT 1
Return the type of transaction with the highest total amount.
[Schema (values) (types)]: | customers_card_transactions |  Accounts : account_id (text) , customer_id (number) , account_name (number) , other_account_details (text) | Customers : customer_id (text) , customer_first_name (number) , customer_last_name (number) , customer_address (text) , customer_phone (text) , customer_email (number) , other_customer_details (text) | Customers_Cards : card_id (text) , customer_id (number) , card_type_code (number) , card_number (text) , date_valid_from (text) , date_valid_to (number) , other_card_details (text) | Financial_Transactions : transaction_id (text) , previous_transaction_id (number) , account_id (number) , card_id (text) , transaction_type (text) , transaction_date (number) , transaction_amount (text) , transaction_comment (text) , other_transaction_details (text);