CREATE TABLE orders (order_id VARCHAR)
CREATE TABLE order_items (order_id VARCHAR)

What is the id of the order which has the most items?
SELECT T1.order_id FROM orders AS T1 JOIN order_items AS T2 ON T1.order_id = T2.order_id GROUP BY T1.order_id ORDER BY COUNT(*) DESC LIMIT 1
SELECT "T1"."order_id" FROM "orders" AS "T1" JOIN "order_items" AS "T2" ON "T1"."order_id" = "T2"."order_id" GROUP BY "T1"."order_id" ORDER BY COUNT(*) DESC NULLS LAST LIMIT 1