CREATE TABLE Customers_cards (card_type_code VARCHAR2, customer_id VARCHAR2)

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"