How can I show order quantity relative to cities where customers in cities have at least three orders?

Question

I use this SQL statement to be able to join three tables (Order1, Order2, and Customers) to show the order quantity for each customer from each city they're addressed to. But how can I show the rows of order quantities for customers in cities who have done at least three orders? In other words I'm trying to aggregate on the cities connected to the customers who have done more than three orders.

Table structures:

• Customers has the columns CustomerNr, City Name
• Order1 has the columns Ordernr, CustomerNr
• Order2 has the columns Ordernr, Order quantity

The SQL statement so far:

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

Answer 1

What you're missing are aggregate functions. To get the sum of a column, you can use SUM(columnName) in your select statement. To get proper results, you will have to group by a field as well. In this case, you want the sum per customer so you can do something like this:

SELECT c.customerNumber, c.name, SUM(o2.quantity) AS totalQuantity
FROM customers c
JOIN order1 o1 ON o1.customerNumber = c.customerNumber
JOIN order2 o2 ON o2.orderNumber = o1.orderNumber
GROUP BY c.customerNumber, c.name;

To filter on an aggregate condition, you need to add a having clause. Here, you can require that each group have a minimum of 3 rows:

SELECT c.customerNumber, c.name, SUM(o2.quantity) AS totalQuantity
FROM customers c
JOIN order1 o1 ON o1.customerNumber = c.customerNumber
JOIN order2 o2 ON o2.orderNumber = o1.orderNumber
GROUP BY c.customerNumber, c.name
HAVING COUNT(*) >= 3;

Here is an SQL Fiddle with some dummy data that I tested with.

Answer 2

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