12.17.8 Spatial Operator Functions

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

In addition, Section 12.17.7, “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)

These spatial operator functions are available:

• 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)) |
  +--------------------------------------+