CREATE TABLE customer_orders (order_id NUMBER, customer_id NUMBER, store_id NUMBER, order_date TIME, planned_delivery_date TIME, actual_delivery_date TIME, other_order_details CLOB)
CREATE TABLE customers (customer_id CLOB, address_id NUMBER, customer_name CLOB, customer_phone CLOB, customer_email_address CLOB, other_details CLOB)
CREATE TABLE invoices (invoice_id NUMBER, order_id NUMBER, payment_method_code CLOB, product_id NUMBER, order_quantity CLOB, other_item_details CLOB, order_item_id NUMBER)
CREATE TABLE order_items (order_item_id NUMBER, order_id NUMBER, product_id NUMBER, order_quantity CLOB, other_item_details CLOB)
CREATE TABLE bookings_services (order_id NUMBER, product_id NUMBER)
CREATE TABLE clients (client_id NUMBER, address_id NUMBER, customer_email_address CLOB, customer_name CLOB, customer_phone CLOB, other_details CLOB)
CREATE TABLE performers (performer_id NUMBER, address_id NUMBER, customer_name CLOB, customer_phone CLOB, customer_email_address CLOB, other_details CLOB)
CREATE TABLE services (service_id NUMBER, service_type_code CLOB, workshop_group_id NUMBER, product_description CLOB, product_name CLOB, product_price NUMBER, other_product_service_details CLOB)
CREATE TABLE marketing_regions (marketing_region_code CLOB, marketing_region_name CLOB, marketing_region_descriptrion CLOB, other_details CLOB)
CREATE TABLE invoice_items (invoice_item_id NUMBER, invoice_id NUMBER, order_id NUMBER, order_item_id NUMBER, product_id NUMBER, order_quantity NUMBER, other_item_details CLOB)
CREATE TABLE stores (store_id CLOB, address_id NUMBER, marketing_region_code CLOB, store_name CLOB, store_phone CLOB, store_email_address CLOB, other_details CLOB)
CREATE TABLE ref_payment_methods (payment_method_code CLOB, payment_method_description CLOB)
CREATE TABLE addresses (address_id CLOB, line_1 CLOB, line_2 CLOB, city_town CLOB, state_county CLOB, other_details CLOB)
CREATE TABLE bookings (booking_id NUMBER, customer_id NUMBER, workshop_group_id CLOB, status_code CLOB, store_id NUMBER, order_date TIME, planned_delivery_date TIME, actual_delivery_date TIME, other_order_details CLOB)
CREATE TABLE ref_service_types (service_type_code CLOB, parent_service_type_code CLOB, service_type_description CLOB)
CREATE TABLE products (product_id CLOB, product_name CLOB, product_price NUMBER, product_description CLOB, other_product_service_details CLOB)
CREATE TABLE drama_workshop_groups (workshop_group_id NUMBER, address_id NUMBER, currency_code CLOB, marketing_region_code CLOB, store_name CLOB, store_phone CLOB, store_email_address CLOB, other_details CLOB)
CREATE TABLE performers_in_bookings (order_id NUMBER, performer_id NUMBER)

What is the payment method code used by the most orders?
SELECT payment_method_code FROM invoices GROUP BY payment_method_code ORDER BY COUNT(*) DESC LIMIT 1
SELECT "payment_method_code" FROM "invoices" GROUP BY "payment_method_code" ORDER BY COUNT(*) DESC FETCH FIRST 1 ROWS ONLY