Question

我正在做一些SQL语句的例子。让我们说我有这些表：

会员

MemberID | LastName | FirstName | Birthday | CampusAddress | CampusPhone

FoodItem

FoodItemID | FoodItemName | FoodItemType

_Order

OrderID | MemberID | FoodItemType

订单行

OrderID | FoodItemID | Quantity | Price

我的目标是列出在同一订单上同时订购牛肉（FoodItemID =＆＃34; 101＆＃34;）和鸡肉（FoodItemID =＆＃34; 102＆＃34;）的所有会员（memberID，lastname）。

我写了两个声明：

SELECT m.memberid, 
       m.lastname 
FROM   member AS m, 
       _order AS o, 
       orderline AS ol 
WHERE  m.memberid = o.memberid 
       AND o.orderid = ol.orderid 
       AND ol.fooditemid = '101' 
       AND ol.fooditemid IN (SELECT ol.fooditemid 
                             FROM   member AS m, 
                                    _order AS o, 
                                    orderline AS ol 
                             WHERE  m.memberid = o.memberid 
                                    AND o.orderid = ol.orderid 
                                    AND ol.fooditemid = '102')

并且

SELECT m.memberid, 
       m.lastname 
FROM   member AS m, 
       _order AS o, 
       orderline AS ol 
WHERE  m.memberid = o.memberid 
       AND o.orderid = ol.orderid 
       AND ( ol.fooditemid = '101' 
              OR ol.fooditemid = '102' )

哪一个是对的？

Answer 1

我会说没有一个是正确的，因为你正在进行隐式连接，这在SQL中已经过时了。

SELECT DISTINCT o.MemberID
                FROM Order o
                WHERE EXISTS (SELECT 1 
                                FROM OrderItem o1 
                               WHERE o.orderid = o1.orderid AND       
                                     o1.fooditemid='101') 
                AND
                      EXISTS (SELECT 1 
                                FROM OrderItem o2 
                               WHERE o.orderid = o2.orderid AND       
                                     o2.fooditemid='102')

这应该有效，但不确定，如果这是最好的方法。

Answer 2

您的所有查询都无法满足您的要求。这应该工作：

select distinct Member.LastName,
                Member.FirstName,
                o1.FoodItemID,
                o2.FoodItemID
    from _order
    inner join _order as o1
        on _order.OrderID = o1.OrderID
    inner join _order o2
        on _order.OrderID = o2.OrderID
    inner join Member
        on _order.MemberID = Member.MemberID
    where o1.FoodItemID = '101'
        and o2.FoodItemID = '102'

在这里，您将只获得包含食物项ID并且它们位于投影视图中的订单，以便您可以使用where子句过滤它们。我不知道你是否动态创建了你的查询。

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