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

With T as (
    SELECT  Customer,
            [Transaction Year],
            sum(Amount) AS TransactionAmount
    FROM Transactions
    GROUP BY Customer, [Transaction Year]
), C AS 
    SELECT  Customer,
            [Claim Year],
            sum(Amount) as ClaimAmount
    FROM Claims
    GROUP BY Customer, [Claim Year]
)
SELECT  T.Customer,
        [Transactions Year],
        TransactionAmount,
        ClaimAmount
FROM    T
   LEFT JOIN C ON C.Customer = T.Customer
      AND [Transactions Year] = [Claim Year]