Could we use Two or more diffrent Table in one Report Table in local report

I'm designing a report which currentely includes three tables: dataset1_sale_details, dataset1_target, &dataset1_commisions.

I need to make a very simple comparsion between two table, where the sum of sale_details of a product id will be compare with target of a product id, and then when it's bigger then the diffrence will bemultiple with commisions of the product id.
I know how to write an expression for fields which are on the same table, but I don't know how to use expression for field in diffrent table.

If someone has an idea, suggestion or sample codefor such expression it will be great!

Thanks in advance!

[1150 byte] By [lemune] at [2008-1-7]