Below is an context that describes a sql query, paired with an question that provides further information. Write an answer that appropriately completes the request.  # noqa: E501
### Context:
CREATE TABLE order_items (product_id VARCHAR)
### Question:
What is the id of the product that was ordered the most often?
### Answer:
SELECT product_id FROM order_items GROUP BY product_id ORDER BY COUNT(*) DESC LIMIT 1