What are the different card types, and how many transactions have been made with each?
SELECT T2.card_type_code ,  count(*) FROM Financial_transactions AS T1 JOIN Customers_cards AS T2 ON T1.card_id  =  T2.card_id GROUP BY T2.card_type_code
different card types refers to DISTINCT(T2.card_type_code); number of transactions made with each refers to COUNT(T1.transaction_id);