[postgis-users] clip a polygon layer with another polygon later

[Puneet Kishor]

Given

	CREATE TABLE outline (
		id,
		the_geom
	);

and

	CREATE TABLE polys (
		id,
		the_geom
	);

I want to update polys.the_geom with only those that are clipped by "outline". That is, all the "polys" that are completely within the ouline.the_geom rows should remain intact, all those that are intersected should only have there overlapping portion retained, and all that are outside outline.the_geom should be set to null.

I am unable to figure out how to do this in one query. The following doesn't work

	UPDATE 	polys
	SET 	the_geom = ST_Intersection(o.the_geom, p.the_geom)
	FROM 	outline o JOIN 
		polys p ON ST_Intersects(o.the_geom, p.the_geom)
	WHERE	ST_Intersects(o.the_geom, p.the_geom) = 'true';



--
Puneet Kishor