Ben, Perhaps I'm missing something -- but why don't you just put an ORDER BY in your DISTINCT ON to control which one is returned?
SELECT DISTINCT ON (pic) lga_name09 as shire, propname as name, pic FROM lga l JOIN qldproperties q ON st_intersects(q.the_geom, l.gda_geom) AND l.gid in (245,247,252,254,258,259,275,279,289,297) ORDER BY pic, ST_Area(st_intersection(q.the_geom, l.gda_geom)) DESC Hope that helps, Regina http://www.postgis.us -----Original Message----- From: postgis-users-boun...@postgis.refractions.net [mailto:postgis-users-boun...@postgis.refractions.net] On Behalf Of Ben Madin Sent: Monday, June 06, 2011 5:36 PM To: PostGIS Users Discussion Subject: [postgis-users] Selecting Unique polygons with st_intersects() G'day all, I've just realised that in selecting a group of properties by the local government area they reside in, I end up with duplicates = some properties span shire boundaries. I've used st_intersects, because some properties cover boundaries, so I need those that are within and may be partially without the boundary. I'm sure I'm not the first person to have this problem, but I was wondering if anyone has any insights into the most efficient way to choose a unique listing of property and shire. I was wondering about taking the area of the property still within the local government area, and choosing the row with the biggest value... Although I have unique property identifiers, I can't be sure of not getting a ridiculous answer - an erroneously small amount of a property in a shire etc. My current query looks a bit like : SELECT DISTINCT ON (pic) lga_name09 as shire, propname as name, pic FROM lga l JOIN qldproperties q ON st_intersects(q.the_geom, l.gda_geom) AND l.gid in (245,247,252,254,258,259,275,279,289,297); but I need to do better than using SELECT DISTINCT ON. cheers Ben _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
