11.5.2.1 General Geometry Property Analysis Functions

The functions listed in this ssection do not restrict their argument and accept a geometry value of any type.

GeometryType(g)

Returns as a string the name of the geometry type of which the geometry instance g is a member. The name will correspond to one of the instantiable Geometry subclasses.

mysql> SELECT GeometryType(GeomFromText('POINT(1 1)'));
+------------------------------------------+
| GeometryType(GeomFromText('POINT(1 1)')) |
+------------------------------------------+
| POINT                                    |
+------------------------------------------+

Dimension(g)

Returns the inherent dimension of the geometry value g. The result can be -1, 0, 1, or 2. (The meaning of these values is given in section 11.2.2 Class Geometry.)

mysql> SELECT Dimension(GeomFromText('LineString(1 1,2 2)'));
+------------------------------------------------+
| Dimension(GeomFromText('LineString(1 1,2 2)')) |
+------------------------------------------------+
|                                              1 |
+------------------------------------------------+

SRID(g)

Returns an integer indicating the Spatial Reference System ID for the geometry value g.

mysql> SELECT SRID(GeomFromText('LineString(1 1,2 2)',101));
+-----------------------------------------------+
| SRID(GeomFromText('LineString(1 1,2 2)',101)) |
+-----------------------------------------------+
|                                           101 |
+-----------------------------------------------+

Envelope(g)

Returns the Minimum Bounding Rectangle (MBR) for the geometry value g. The result is returned as a polygon value.

mysql> SELECT AsText(Envelope(GeomFromText('LineString(1 1,2 2)')));
+-------------------------------------------------------+
| AsText(Envelope(GeomFromText('LineString(1 1,2 2)'))) |
+-------------------------------------------------------+
| POLYGON((1 1,2 1,2 2,1 2,1 1))                        |
+-------------------------------------------------------+

The polygon is defined by the corner points of the bounding box:

POLYGON((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY))

The OpenGIS specification also defines the following functions, which MySQL does not yet implement:

Boundary(g)

Returns a geometry that is the closure of the combinatorial boundary of the geometry value g.

IsEmpty(g)

Returns 1 if the geomtry value g is the empty geometry, 0 if it is not empty, and -1 if the argument is NULL. If the geometry is empty, it represents the empty point set.

IsSimple(g)

Returns 1 if the geometry value g has no anomalous geometric points, such as self intersection or self tangency. IsSimple() returns 0 if the argument is not simple, and -1 if it is NULL. The description of each instantiable geometric class given earlier in the chapter includes the specific conditions that cause an instance of that class to be classified as not simple.

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