CREATE TABLE customer_orders (customer_id VARCHAR, order_id VARCHAR)
CREATE TABLE order_items (order_id VARCHAR, order_quantity INT)
CREATE TABLE customers (payment_method VARCHAR, customer_name VARCHAR, customer_id VARCHAR)

What is the payment method of the customer that has purchased the least quantity of items?
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
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") NULLS FIRST LIMIT 1