Table schema:
CREATE TABLE order_items (order_id VARCHAR, order_quantity INTEGER); CREATE TABLE customers (payment_method VARCHAR, customer_name VARCHAR, customer_id VARCHAR); CREATE TABLE customer_orders (customer_id VARCHAR, order_id VARCHAR)

Question: What is the payment method of the customer that has purchased the least quantity of items?
SQL: SELECT t1.payment_method FROM customers AS t1 JOIN customer_orders AS t2 ON t1.customer_id = t2.customer_id JOIN order_items AS t3 ON t2.order_id = t3.order_id GROUP BY t1.customer_name ORDER BY SUM(t3.order_quantity) LIMIT 1