This illustration shows the order_items master table and the inventories master table. The order_items table has a primary key that includes both the order_id column and the line_item_id column. The inventories table has a primary key that includes both the product_id column and the warehouse_id column. The product_id column exists in both tables. There is a many to many relationship between the order_items and inventories tables because many order items can be available in many inventories.