This question is very similar to #131823, that is not solved yet.
I want to merge several points like bus stops into one single point in the centroid of all selected points. The bus stops have a attribute "name". The problem is that these names are not necessarily unique worldwide, so there needs to be a spatial limit if several points can be combined to a MultiPoint. This done, we can calculate the centroid with PostGIS: http://www.postgis.org/docs/ST_Centroid.html
How can we solve this problem with Postgis functions?