Question

I have the following database schema (Microsoft SQL Server Compact Edition):

How can I select a customer record or customer records, with an additional field containing the total book value of all vehicles owned by the customer? I tried something like:

SELECT Customers.ID, Customers.FirstName, ..., SUM(Receipts.BookValue) FROM Customers INNER JOIN Vehicles ON Vehicles.CustomerID = Customers.ID INNER JOIN Receipts ON Receipts.VehicleID = Vehicles.ID;

but was unsuccessful. I’m thinking I might need a GROUP BY or something?

Answer 1

SELECT Customers.ID, SUM(Receipts.BookValue) FROM Customers INNER JOIN Vehicles ON Vehicles.CustomerID = Customers.ID INNER JOIN Receipts ON Receipts.VehicleID = Vehicles.ID
GROUP BY Customers.ID

Also please note that when using aggregates like Sum you must ensure that all additional fields are part of an aggregate type or in the GROUP BY list.

Sometimes its easier to make 2 queries, the inner query does all the summing and the rest displays additional info like this:

SELECT t.TheSum,
       c.ID,
       c.CompanyName FROM
   ( SELECT Customers.ID as TheID, 
           SUM(Receipts.BookValue) AS TheSum
    FROM Customers INNER JOIN Vehicles ON Vehicles.CustomerID = Customers.ID INNER JOIN    Receipts ON Receipts.VehicleID = Vehicles.ID
    GROUP BY Customers.ID
) t
INNER JOIN Customers c ON c.ID = t.TheID