-- Database schema
| Accounts : account_id [ INT ] primary_key , customer_id [ INT ] , account_name [ TEXT ] , other_account_details [ TEXT ] | Customers : customer_id [ INT ] primary_key , customer_first_name [ TEXT ] , customer_last_name [ TEXT ] , customer_address [ TEXT ] , customer_phone [ TEXT ] , customer_email [ TEXT ] , other_customer_details [ TEXT ] | Customers_Cards : card_id [ INT ] primary_key , customer_id [ INT ] , card_type_code [ TEXT ] , card_number [ TEXT ] , date_valid_from [ TEXT ] , date_valid_to [ TEXT ] , other_card_details [ TEXT ] | Financial_Transactions : transaction_id [ INT ] , previous_transaction_id [ INT ] , account_id [ INT ] Financial_Transactions.account_id = Accounts.account_id , card_id [ INT ] Financial_Transactions.card_id = Customers_Cards.card_id , transaction_type [ TEXT ] , transaction_date [ TEXT ] , transaction_amount [ INT ] , transaction_comment [ TEXT ] , other_transaction_details [ TEXT ] |
-- -- Return the code of the card type that is most common.

SELECT card_type_code FROM Customers_cards GROUP BY card_type_code ORDER BY count(*) DESC LIMIT 1;