Modeling SQL Query to add statistical data to a geometry

Hello I have a problem modeling a SQL Query for the following problem:

I have two tables with geometries from Shapefile der Verwaltungsgrenzen (WGS84). Of intrest for me are Kreise and Gemeinden, whereas a Gemeinde is always part of the bigger Kreis. Each Kreis has a unique id called rs which is exactly 5 chars long, where as the unique id of Gemeinden is 12 chars long and the first 5 chars are equal to a Kreis id.

For some Gemeinden in de_commuter_gemeinden I have statistical data as well as for most of the Kreise in de_commuter_kreise. These data describes how many commuter there are.

For each geometry with statistical data I have to create random points within the geometry. For the Gemeinden it is no problem, since the are a subset of a Kreis and I can select the geometry from the table de_shp_gemeinden.geom. But for the Kreise I have to substract the already generated points as well as the geometry they were created in. My current SQL for this is:

SELECT 
  k.rs, k.gen, st_difference(k.geom, (
    SELECT ST_Union(geom) AS geom 
    FROM de_shp_gemeinden g, de_commuter_gemeinden c 
    WHERE c.rs ~ ('^' || k.rs) AND c.rs = g.rs)
    ) AS geom 
FROM de_shp_kreise k 

Which looks like this

Now I need a smart way on calculating the statistical data for each Kreis, meaning get the data of commuters for the Kreis and substracting all the data from it subset Gemeinden.

Could anyone point me in the right directions? If you need more info please ask. Thank you.