nested Sum and DistinctCount error. Help!

Hi I'm really a newb at CR, (started this monday), and I wanted to do a statement like this:

Sum(DistinctCount({db_column3}, {db_column2}), {db_column1})

where the db_column1 can have many db_column2, and db_column2 can have many (repetitious) db_column3.

I'm assuming this wont work because one can't have a sum of a count, but what can I write that is to this effect? thanks a lot in advance!

Tom

Edit, i just tried doing Sum(DistinctCount(blah.. on something trivial, and it worked. So perhaps there are some constraints on the Cond in Sum(blah, Cond) that I don't know. Any enlightenment is much appreciated!>>i just tried doing Sum(DistinctCount(blah.. on something trivial, and it worked.

Then where are you struggling?