Sql Problem

Is there any sql guru's out there?

If so can you let me know if it is possible to do the following:

SELECT tblSector.Sector, Sum(tblBuy.tempValue) AS SumOftempValue, Sum(tblBuy.TotalCost) AS SumOfTotalCost" & _
                &n bsp;           " FROM ((tblBuy INNER JOIN tblInstruments ON tblBuy.Instrument_ID = tblInstruments.Instrument_ID) INNER JOIN tblCodes ON tblBuy.Code_ID = tblCodes.Code_ID) LEFT JOIN tblSector ON tblCodes.Sector_ID = tblSector.Sector_ID" & _
                &n bsp;           " Where (((tblBuy.TradeOpen) = True)) GROUP BY tblSector.Sector;"

In the above sql SumOftempValue can have a null value and whenit does I want the sql to use SumOfTotalCost.

So if SumOftempValue = null then SumOfTotalCost. Can this be in one field in the sql so i do not need to check it in the code?

Any idea? Does it make sense?