大师!我被困住了。目录项目的价格取决于其数量。这里的表格示例:
items: just item definitions
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item_id | item_title
-------------------------
1 | "sample item"
2 | "another item"
items_prices: prices dependent to item quantity.
Less price taken for more quantity of item
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item_id | quantity | price
----------------------------
1 | 1 | 100
1 | 5 | 80
1 | 10 | 60
2 | 1 | 120
2 | 3 | 100
cart
-------------------
item_id | quantity
-------------------
1 | 20
2 | 2
是否可以通过单个查询获得当前的购物车成本?