如何连接三个表，其中一个表与另外两个表的ID不匹配？

Question

我试图弄清楚如何用适当的SQL语句解决这个问题： “向客户至少发出三份订单的城市中的所有客户销售了多少，结果应显示城市名称和这些客户的总订单数量“。

问题是我有三个表可以使用：Order1，Order2和Customers。这些表的结构是这样的：

Order1 has the attributes: Ordernr, CustomerNr. 
Order2 has the attributes: Ordernr, Order quantity. 
Customers has the attributes: CustomerNr, Customer name, City name. 

有了这个代码，我可以加入三个表，以便能够显示订单数量从每个城市每一位客户。但是，如何在至少有三个订单的城市中显示订单数量？

SELECT Customers.CustomerNr, CityName, Order2.OrderQuantity 
FROM Order1 INNER JOIN Order2 
    ON Order1.ordernr = Order2.ordernr JOIN Customers 
    ON Customers.CustomerNr = Order1.CustomerNr 

Answer 1

试试这个：

SELECT c.CityName, SUM(o2.OrderQuantity) OrderQuantity 
FROM Order1 o1 
INNER JOIN (SELECT o1.CustomerNr 
      FROM Order1 o1 
      GROUP BY o1.CustomerNr 
      HAVING COUNT(o1.Ordernr) >= 3 
      ) AS a ON o1.CustomerNr = a.CustomerNr 
INNER JOIN Order2 o2 ON o1.Ordernr = o2.Ordernr 
INNER JOIN Customers c ON o1.CustomerNr = c.CustomerNr 
GROUP BY c.CityName 

Answer 2

这个查询将返回城市的列表有至少3个数量级：

select CityName 
    from Customers c 
     join Order1 o1 on c.CustomerNr = o1.CustomerNr 
     join Order2 o2 on o1.OrderNr = o2.OrderNr 
group by c.CityName having Sum(o2.OrderQuantity) > 3 

现在你想从这些城市从客户的所有订单（我从表Order1假设）

select o1.* 
    from Order1 o1 joib Customer c on c.CustomerNr = o.CustomerNr 
where c.CityName in (
    select CityName 
    from Customers c 
     join Order1 o1 on c.CustomerNr = o1.CustomerNr 
     join Order2 o2 on o1.OrderNr = o2.OrderNr 
group by c.CityName having Sum(o2.OrderQuantity) > 3 
) 

希望这会有所帮助