SELECT card_type_code ,  count(DISTINCT customer_id) FROM Customers_cards GROUP BY card_type_code
Show all card type codes and the number of customers holding cards in each type.
SELECT card_type_code , count ( DISTINCT customer_id ) FROM Customers_cards GROUP BY card_type_code
select card_type_code , count ( distinct customer_id ) from customers_cards group by card_type_code
Show all card type codes and the number of customers holding cards in each type .