Show the product ids and the number of unique orders containing each product.
SELECT product_id ,  count(DISTINCT order_id) FROM Order_items GROUP BY product_id
SELECT product_id ,  count(DISTINCT order_id) FROM Order_items GROUP BY product_id
[{'product_id': 3, 'count(DISTINCT order_id)': 1}, {'product_id': 4, 'count(DISTINCT order_id)': 2}, {'product_id': 5, 'count(DISTINCT order_id)': 2}, {'product_id': 7, 'count(DISTINCT order_id)': 1}, {'product_id': 9, 'count(DISTINCT order_id)': 1}, {'product_id': 10, 'count(DISTINCT order_id)': 1}, {'product_id': 12, 'count(DISTINCT order_id)': 1}, {'product_id': 13, 'count(DISTINCT order_id)': 1}, {'product_id': 14, 'count(DISTINCT order_id)': 3}, {'product_id': 15, 'count(DISTINCT order_id)': 1}]