The sales team wants to understand the average selling price of each product across all units sold.
The order_items table records the product, quantity sold (qty), and selling price per unit (unit_price) for each order item.
For each product, calculate the average selling price across all units sold. The average should account for the quantity sold at each price, rather than treating every order item equally.
For example, an order item containing 3 units should contribute three times as much to the average as an order item containing 1 unit.
Ordering: Sort by product_id in ascending order.