so the sample data and expected output?

If you're doing everything at rendering just pull everything, then group by columnA, add your calculations on columns b,c or whatnot. It's about the best I can do without something more concrete from you.

I'm not sure at rendering is the best place to do all of your aggregation but YMMV depending on how much data you're talking about. No need to pull 1000 rows across the network just to aggregate them down to 5 when you can do that at the database and pull your 5 records. While SSRS does a pretty good job of this, the larger the dataset, the longer it takes to get to the SSRS server across the network, and thus the longer the user waits on rendering.

-Luke.