用mysql查詢計算價格

Question

大師！我被卡住了。目錄項目的價格取決於其數量。這裏的表格的例子：

items: just item definitions 
------------------------- 
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 
---------------------------- 
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 

是否有可能通過單個查詢獲得當前購物車的成本？

Answer 1

select sum(x.totalcost) 
from (
    select c.item_id, c.quantity * ip.price as totalcost 
    from cart c 
    join items_prices ip 
     on c.item_id = ip.item_id 
    left join items_prices ip2 
     on ip2.quantity > ip.quantity 
     and c.quantity >= ip2.quantity 
    where c.quantity >= ip.quantity 
     and ip2.quantity is null 
) x 

的加入放回items_price再次讓我們過濾掉任何情況下，有這仍然符合我們的標準，更大的量。這應該越來越接近我們之後的東西

Answer 2

對於項目編號1 23數量：

$item_id = 1; 
$quantity = 23; 

$sql = 'SELECT price * '.$quantity.' 
    FROM items_prices 
    WHERE item_id = '.$item_id.' 
     AND quantity <= '.$quantity.' 
    ORDER BY quantity DESC 
    LIMIT 1';