Below is the list of tables from the database along with their columns and datatype. Need to generate an accurate SQL query for the question.
Only use the columns present in the respective table. Only use the columns which are required to generate the SQL query. Only use table joins if required and not otherwise.

### Tables ###
TABLE: 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.

### Accurate SQL ###

SELECT card_type_code, COUNT(DISTINCT customer_id) FROM Customers_cards GROUP BY card_type_code