ST_Difference — Returns a geometry that represents that part of geometry A that does not intersect with geometry B.
geometry ST_Difference(geometry geomA, geometry geomB
Returns a geometry that represents that part of geometry A that does not intersect with geometry B. One can think of this as GeometryA - ST_Intersection(A,B). If A is completely contained in B then an empty geometry collection is returned.
Order matters. B - A will always return a portion of B
Performed by the GEOS module
This method implements the OpenGIS Simple Features Implementation Specification for SQL 1.1. s22.214.171.124
This method implements the SQL/MM specification. SQL-MM 3: 5.1.20
This function supports 3d and will not drop the z-index. However it seems to only consider x y when doing the difference and tacks back on the Z-Index
Safe for 2D. This is same geometries as what is shown for st_symdifference
SELECT ST_AsText( ST_Difference( 'LINESTRING(50 100, 50 200)'::geometry, 'LINESTRING(50 50, 50 150)'::geometry ) ); st_astext --------- LINESTRING(50 150,50 200)
When used in 3d doesn't quite do the right thing.
select ST_AsEWKT( ST_Difference( 'MULTIPOINT(-118.58 38.38 5,-118.60 38.329 6,-118.614 38.281 7)' :: geometry, 'POINT(-118.614 38.281 5)' :: geometry ) ); st_asewkt --------- MULTIPOINT(-118.6 38.329 6,-118.58 38.38 5)