如何連接三個表，其中一個表與另外兩個表的ID不匹配？

Question

我試圖弄清楚如何用適當的SQL語句解決這個問題： 「向客戶至少發出三份訂單的城市中的所有客戶銷售了多少，結果應顯示城市名稱和這些客戶的總訂單數量「。

問題是我有三個表可以使用：Order1，Order2和Customers。這些表的結構是這樣的：

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

有了這個代碼，我可以加入三個表，以便能夠顯示訂單數量從每個城市每一位客戶。但是，如何在至少有三個訂單的城市中顯示訂單數量？

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

試試這個：

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

這個查詢將返回城市的列表有至少3個數量級：

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 

現在你想從這些城市從客戶的所有訂單（我從表Order1假設）

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 
) 

希望這會有所幫助