SELECT card_type_code ,  count(DISTINCT customer_id) FROM Customers_cards GROUP BY card_type_code
What are the different card type codes, and how many different customers hold 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
What are the different card type codes , and how many different customers hold each type ?