CREATE TABLE products (product_details VARCHAR, product_id VARCHAR)
CREATE TABLE order_items (product_id VARCHAR)

Which product has been ordered most number of times?
SELECT t2.product_details FROM order_items AS t1 JOIN products AS t2 ON t1.product_id = t2.product_id GROUP BY t1.product_id ORDER BY COUNT(*) DESC LIMIT 1
SELECT "t2"."product_details" FROM "order_items" AS "t1" JOIN "products" AS "t2" ON "t1"."product_id" = "t2"."product_id" GROUP BY "t1"."product_id" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1