Summing #s In A Query

Hello,

I'm trying to sum numbers in a query. The table that Im summing from is set up as follows. The 1st column is just an autonumber. The 2nd column is an individuals. The third column is a date. The fourth column is a transaction (Buy or Sell). The fifth column is an amount (in $). and the 6th column is an Account (RRSP or RESP).

I would like to sum all the buys for each individual for each account. However I'm finding this difficult to do. Here is what i have so far. It sums all the 'buys' for the entire table.

SELECT Transactions.[Customer ID], DSum("Amount","Transactions","Transactions.Transaction=1") AS Expr1, Accounts.[Account Type ID]
FROM Transactions INNER JOIN Accounts ON Transactions.[Account ID] = Accounts.[Account ID];

Thanks very much,

C