Calculate Weighted Aggregation between Two Tables

I have global national statistics (GDP, Fish Catch etc.), which I would like to aggregate to and calculate per Capita data for given regions (Africa, Europe...) and subregions (Western Africa, ....) on-the-fly.

From a statistical point of view it seems best to use something like this:

    given that the variable is "v" and the Total Population table is "tp":

   (country_1_of_region_1_for_v / country_1_of_region_1_for_tp) + (country_2_of_region_1_for_v / country_2_of_region_1_for_tp) + ....

   ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

                                                            (sum_countries_all_of_region_1_for_v / sum_countries_all_of_region_1_for_tp)

    and then same thing for the other regions (in my case a total of 6 regions).

I have a table of countries with a field country_id and region_id (which specifies to which region this country belongs).

I have the table for the variable with a field country_id, which has kind of a "Excel design", i.e. columns for each year.

And I have a table of regions with a field region_id.

It seems like a rather complicated thing to do, as one needs first to get all the countries of a specific region for both tables (variable & pop_total), have the SUM(of all countries) available and run the mathematical procedure... 

Could someone give me a hint how this could be achieved?

Thanks for any help!

Stef