What is the card type with the most number of cards?
select card_type_code from customers_cards group by card_type_code order by count ( * ) desc limit 1
And the one with the least number of cards?
select card_type_code from customers_cards group by card_type_code order by count ( * ) limit 1
How many cards are credit cards?
select count ( * ) from customers_cards where card_type_code = "Credit"
How many customers have credit cards?
select count ( * ) from customers_cards where card_type_code = "Credit"