Generate Reports From Database Or Gridview?

I'm looking for guidance of how to do some things for my application, I cannot resolve how to do the following:I have users and categories in a database, then I have a Master Table where these users combine with the categories creating rows, in this table there are other columns too.So for instance,

Master Table

User1 | Category2 | Date | Cost
john mainteinance nov1988 5000
along with more columns.

I managed to create the other tables so my application can capture this data, can add or erase users or categories, and everything still works great.The problem is that I need to generate reports from this Master Table, so for instance if I select john from a dropdownlist, I need to generate a report with all of its sells totals separated in categories, the thing is, I could just make a query asking for that, but the problem is that like I said you can add new users, new categories, or erase etc... so I just cannot get the logic to build something like a gridview and get what I want just passing the user name selected, and automatically getting me the total of all categories, no matter if later they add one, still comes back with the total of the new one, or if they erase one it will not display it etc...

If you could give me some general info of how to do it it would be appreciated, I'm thinking computed columns but I just don't know how to calculate ti.