Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE Customers_cards (card_type_code VARCHAR, customer_id VARCHAR)
### Question:
Show all card type codes and the number of customers holding cards in each type.
### Answer:
SELECT card_type_code, COUNT(DISTINCT customer_id) FROM Customers_cards GROUP BY card_type_code