CREATE TABLE order_items (product_id VARCHAR2, order_id VARCHAR2)
CREATE TABLE orders (customer_id VARCHAR2, order_id VARCHAR2)
CREATE TABLE products (product_name VARCHAR2, product_id VARCHAR2)
CREATE TABLE customers (customer_name VARCHAR2, customer_id VARCHAR2)

List the names of the customers who have once bought product 'food'.
SELECT T1.customer_name FROM customers AS T1 JOIN orders AS T2 JOIN order_items AS T3 JOIN products AS T4 ON T1.customer_id = T2.customer_id AND T2.order_id = T3.order_id AND T3.product_id = T4.product_id WHERE T4.product_name = "food" GROUP BY T1.customer_id HAVING COUNT(*) >= 1
SELECT "T1"."customer_name" FROM "customers" "T1" CROSS JOIN "order_items" "T3" JOIN "orders" "T2" ON "T1"."customer_id" = "T2"."customer_id" AND "T2"."order_id" = "T3"."order_id" JOIN "products" "T4" ON "T3"."product_id" = "T4"."product_id" AND "T4"."product_name" = "food" GROUP BY "T1"."customer_id" HAVING COUNT(*) >= 1