MySQL的連接表，其中第二表具有關鍵

Question

至少一個NULL字段我有兩個表orders和order_items

ORDER_ITEMS表具有COMPLETION_DATE場，直到該項目完成後，其爲NULL。我只想用至少一個具有NULL Completion_Date的項目返回訂單。

SELECT orders.Order_Number 
    FROM orders 
LEFT JOIN order_items 
    ON (orders.Order_Number = order_items.Order_Number) 
WHERE order_items_Completion_Date IS NULL 

我如何加入那裏是至少一個 NULL字段COMPLETION_DATE爲ORDER_NUMBER關鍵？

Answer 1

如果我理解正確，你就必須把WHERE子句的ON條款那樣：

SELECT orders.Order_Number 
    FROM orders 
LEFT JOIN order_items 
    ON (orders.Order_Number = order_items.Order_Number) 
    AND order_items_Completion_Date IS NULL 

Answer 2

您可能會發現exists做這個工作做得更好：

select o.* 
from orders o 
where exists (select 1 
       from order_items oi 
       where oi.Order_Number = o.Order_Number and 
        oi.Completion_Date is null 
      ); 

雖然您也可以使用join執行此操作，如果訂單中有多個項目的完成日期爲NULL，則該方法可能會導致重複的行。