12.16.8 Spatial Operator Functions

OpenGIS proposes a number of functions that can produce geometries. They are designed to implement spatial operators.

• Buffer(g, d)

  ST_Buffer() and Buffer() are synonyms. For more information, see the description of ST_Buffer().

• ST_Buffer(g, d)

  Returns a geometry that represents all points whose distance from the geometry value g is less than or equal to a distance of d, or NULL if any argument is NULL.

  ST_Buffer() supports negative distances for polygons, multipolygons, and geometry collections containing polygons or multipolygons. For point, multipoint, linestring, multilinestring, and geometry collections not containing any polygons or multipolygons, ST_Buffer() with a negative distance returns NULL.

  ST_Buffer() and Buffer() are synonyms.

• ST_Difference(g1, g2)

  Returns a geometry that represents the point set difference of the geometry values g1 and g2. If any argument is NULL, the return value is NULL.

  mysql> SET @g1 = Point(1,1), @g2 = Point(2,2);
  mysql> SELECT ST_AsText(ST_Difference(@g1, @g2));
  +------------------------------------+
  | ST_AsText(ST_Difference(@g1, @g2)) |
  +------------------------------------+
  | POINT(1 1)                         |
  +------------------------------------+

• ST_Intersection(g1, g2)

  Returns a geometry that represents the point set intersection of the geometry values g1 and g2. If any argument is NULL, the return value is NULL.

  mysql> SET @g1 = ST_GeomFromText('LineString(1 1, 3 3)');
  mysql> SET @g2 = ST_GeomFromText('LineString(1 3, 3 1)');
  mysql> SELECT ST_AsText(ST_Intersection(@g1, @g2));
  +--------------------------------------+
  | ST_AsText(ST_Intersection(@g1, @g2)) |
  +--------------------------------------+
  | POINT(2 2)                           |
  +--------------------------------------+

• ST_SymDifference(g1, g2)

  Returns a geometry that represents the point set symmetric difference of the geometry values g1 and g2, which is defined as:

  g1 symdifference g2 := (g1 union g2) difference (g1 intersection g2)

  Or, in function call notation:

  ST_SymDifference(g1, g2) = ST_Difference(ST_Union(g1, g2), ST_Intersection(g1, g2))

  If any argument is NULL, the return value is NULL.

  mysql> SET @g1 = Point(1,1), @g2 = Point(2,2);
  mysql> SELECT ST_AsText(ST_SymDifference(@g1, @g2));
  +---------------------------------------+
  | ST_AsText(ST_SymDifference(@g1, @g2)) |
  +---------------------------------------+
  | MULTIPOINT(1 1,2 2)                   |
  +---------------------------------------+

• ST_Union(g1, g2)

  Returns a geometry that represents the point set union of the geometry values g1 and g2. If any argument is NULL, the return value is NULL.

  mysql> SET @g1 = ST_GeomFromText('LineString(1 1, 3 3)');
  mysql> SET @g2 = ST_GeomFromText('LineString(1 3, 3 1)');
  mysql> SELECT ST_AsText(ST_Union(@g1, @g2));
  +--------------------------------------+
  | ST_AsText(ST_Union(@g1, @g2))        |
  +--------------------------------------+
  | MULTILINESTRING((1 1,3 3),(3 1,1 3)) |
  +--------------------------------------+

In addition, Section 12.16.7, “Geometry Property Functions”, discusses several functions that construct new geometries from existing ones. See that section for descriptions of these functions:

• ST_Envelope(g)

• StartPoint(ls)

• ST_EndPoint(ls)

• ST_PointN(ls, N)

• ST_ExteriorRing(poly)

• ST_InteriorRingN(poly, N)

• ST_GeometryN(gc, N)