如何联接三个表，其中一个表与其他两个表的ID不匹配？

Question

I'm trying to figure out how to solve this question with a proper SQL statement: "How much has been sold to all customers in cities where a customer has made at least three orders. The result should display the city name and the total order quantity for these customers".

The thing is that I have three tables to work with: Order1, Order2, and Customers. The table structures for these look like this:

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

With this code I can join the three tables to be able to show the order quantity for each customer from each city. But how can I show the order quantity for customers in cities who've at least three orders?

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

This query will return a list of cities with at least 3 orders:

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

Now you want all orders (I suppose from table Order1 ) from customers from these cities

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
 )

Hope this helps

Answer 2

Try this:

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