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
[{'card_type_code': 'Credit', 'count(DISTINCT customer_id)': 6}, {'card_type_code': 'Debit', 'count(DISTINCT customer_id)': 6}]