Very Computer

I need to make a report using MS Data Reports that uses information in a
query to calculate an amount due.  For example, the query only returns
records that don't have a value for this one certain date field, it then
determines the amount due based on the current date and the rest of the
information in that same record.

Here's the basic idea that I need SQL to do:
if [datefield1] is NULL then
    amountdue = ( number of days between today and [datefield2] ) *
feeperday

I guess what I'm asking is for help on how to create a query like this,
getting it into the report shouldn't pose much trouble after that.  Thanks
in advance.

-John