Duplication involving SUM, LEFT JOIN and GROUP BY

Question

I have a problem involving a SUM, LEFT OUTER JOIN and GROUP BY commands, but can\'t figure out where my error is.

I have two tables, one for customer transactions an

Answer 1

The query will be counting the transaction amount twice for 2007 due to the fact you have two claims, so the transaction amount will be counted twice.

i.e. the returned data being used is:

Customer | Transaction Year | Transaction Amount | Claim Amount
----------------------------------------------------------------
A        | 2007             | 100                | 30
A        | 2007             | 100                | 40
A        | 2008             | 80                 |
A        | 2008             | 50                 |
A        | 2009             | 210                | 110

Something like the following, although not pretty, should resolve the problem:

SELECT 
   t.Customer
   ,t.Year
   ,[Transaction Amount] = SUM(t.[Transaction Amount])
   ,[Claim Amount] = c.[Claim Amount]
FROM 
    Transactions t
    LEFT JOIN (
        SELECT 
            Customer
            ,Year
            ,SUM([Claim Amount])
        FROM
           Claims
        GROUP BY
           Customer, Year
    ) c ON c.Customer = t.Customer c.Year = t.Year
GROUP BY t.Customer, t.Year, c.[Claim Amount]