Question

我有两张表orders和order_items

order_items表有一个Completion_Date字段，该字段为NULL，直到该项完成。我只想返回至少有一个具有NULL Completion_Date的项目的订单。

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

如何在Order_Number键的至少一个 NULL Completion_Date字段的哪个地方加入？

Answer 1

您可能会发现exists为此做得更好：

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
             );

虽然您也可以使用join执行此操作，但如果订单包含多个NULL完成日期的项目，则该方法可能会导致重复的行。

Answer 2

如果我理解正确，你只需将WHERE子句放到ON子句中就像那样：

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

MySQL连接表，其中第二个表至少有一个用于键的NULL字段

2 个答案: