Rafal, This question is more appropriate for the PostGIS User's group rather than the PostGIS developer's group.
To answer your question -- We are suspecting you will want to do a SELF JOIN accompanied by a ST_Union, ST_Intersection, and/or possibly a ST_SymDifference. What is unclear to us is If you have 3 polygons - lets say 1,2,3 1 intersects with 2 2 intersects with 3 (but not with 1) What are you expecting to happen here? A) intersection(1,2) count 2, Intersection(2,3) count 2 Or B) You don't allow the same geometry to be counted twice in which case You throw out the whole thing return the empty set Or C) You arbitrarily pick a set and return intersection (1,2) ---- D) Similarly we assume if you have a case where 1,2,3 all intersect then you would return ST_Intersection(ST_Intersection(1,2),3), count 3 ? Or Would you treat as a pairwise intersection E) 1,2,3 x 1,2, 3 intesection resutling in 3 records with count of 2 each 1,2 2,3 1,3 Leo and Regina http://www.postgis.us -----Original Message----- From: postgis-devel-boun...@postgis.refractions.net [mailto:postgis-devel-boun...@postgis.refractions.net] On Behalf Of Rafal Foltynski Sent: Friday, April 23, 2010 11:32 AM To: postgis-de...@postgis.refractions.net Subject: [postgis-devel] St_Intersection on same table Hi, I have a table with many overlying polygons. I want to create a new polygon relation from the intersection of these polygons that has two fields 1. geometry of small polygons - the result of St_Intersection() and 2. count of the number of overlying polygons that each small polygon was created from. How do I create a query to run St_Intersect on the same geometry field in one table? For example: if I have a table with 3 polygons, 2 of them share a small area and the third polygon is disjoint, the resulting polygon relation would only have one small polygon area (shared area between two polygons) and count of 2. Rafal _______________________________________________ postgis-devel mailing list postgis-de...@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-devel _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users