Give the order ids for all orders, as well as the total product quantity in each.
SELECT order_id ,  sum(product_quantity) FROM Order_items GROUP BY order_id
SELECT order_id ,  sum(product_quantity) FROM Order_items GROUP BY order_id
[{'order_id': 1, 'sum(product_quantity)': 12}, {'order_id': 2, 'sum(product_quantity)': 3}, {'order_id': 4, 'sum(product_quantity)': 17}, {'order_id': 10, 'sum(product_quantity)': 4}, {'order_id': 12, 'sum(product_quantity)': 1}, {'order_id': 13, 'sum(product_quantity)': 18}, {'order_id': 14, 'sum(product_quantity)': 1}, {'order_id': 15, 'sum(product_quantity)': 9}]