geometry-expression.ST_Difference(geo2)

The ST_Difference method finds the spatial difference of two geometries. A point is included in the result if it is present in the geometry-expression but not present in geo2.

Unlike other spatial set operations (ST_Union, ST_Intersection, ST_SymDifference), the ST_Difference method is not symmetric: the method can give a different answer for A.ST_Difference( B ) and B.ST_Difference( A ).

• ST_Intersection method for type ST_Geometry
• ST_SymDifference method for type ST_Geometry
• ST_Union method for type ST_Geometry

The following example shows the difference (C) of a square (A) with a circle (B) removed and the difference (D) of a circle (B) with a square (A) removed.

SELECT NEW ST_Polygon( 'Polygon( (-1 -0.25, 1 -0.25, 1 2.25, -1 2.25, -1 -0.25) )' ) AS A
    , NEW ST_CurvePolygon( 'CurvePolygon( CircularString( 0 1, 1 2, 2 1, 1 0, 0 1 ) )' )  AS B
    , A.ST_Difference( B ) AS C
    , B.ST_Difference( A ) AS D

The following picture shows the difference C=A-B and D=B-A as the shaded portion of the picture. Each difference is a single surface that contains all of the points that are in the geometry on the left hand side of the difference and not in the geometry on the right hand side.