11.17.5.2. Geometry Functions

Each function that belongs to this group takes a geometry value as its argument and returns some quantitative or qualitative property of the geometry. Some functions restrict their argument type. Such functions return NULL if the argument is of an incorrect geometry type. For example, Area() returns NULL if the object type is neither Polygon nor MultiPolygon.

11.17.5.2.1. General Geometry Functions

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

  • 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 11.17.2.2, “Class Geometry.

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

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

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

    POLYGON((MINX MINY, MAXX MINY, MAXX MAXY, MINX MAXY, MINX MINY))
    
    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))                        |
    +-------------------------------------------------------+
    
  • GeometryType(g)

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

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

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

    In MySQL, the SRID value is just an integer associated with the geometry value. All calculations are done assuming Euclidean (planar) geometry.

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

The OpenGIS specification also defines the following functions, which MySQL does not 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 geometry 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)

    Currently, this function is a placeholder and should not be used. If implemented, its behavior will be as described in the next paragraph.

    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. (See Section 11.17.2.1, “The Geometry Class Hierarchy”.)

11.17.5.2.2. Point Functions

A Point consists of X and Y coordinates, which may be obtained using the following functions:

  • X(p)

    Returns the X-coordinate value for the Point object p as a double-precision number.

    mysql> SELECT X(POINT(56.7, 53.34));
    +-----------------------+
    | X(POINT(56.7, 53.34)) |
    +-----------------------+
    |                  56.7 |
    +-----------------------+
    
  • Y(p)

    Returns the Y-coordinate value for the Point object p as a double-precision number.

    mysql> SELECT Y(POINT(56.7, 53.34));
    +-----------------------+
    | Y(POINT(56.7, 53.34)) |
    +-----------------------+
    |                 53.34 |
    +-----------------------+
    
11.17.5.2.3. LineString Functions

A LineString consists of Point values. You can extract particular points of a LineString, count the number of points that it contains, or obtain its length.

  • EndPoint(ls)

    Returns the Point that is the endpoint of the LineString value ls.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT AsText(EndPoint(GeomFromText(@ls)));
    +-------------------------------------+
    | AsText(EndPoint(GeomFromText(@ls))) |
    +-------------------------------------+
    | POINT(3 3)                          |
    +-------------------------------------+
    
  • GLength(ls)

    Returns as a double-precision number the length of the LineString value ls in its associated spatial reference.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT GLength(GeomFromText(@ls));
    +----------------------------+
    | GLength(GeomFromText(@ls)) |
    +----------------------------+
    |            2.8284271247462 |
    +----------------------------+
    

    GLength() is a nonstandard name. It corresponds to the OpenGIS Length() function.

  • NumPoints(ls)

    Returns the number of Point objects in the LineString value ls.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT NumPoints(GeomFromText(@ls));
    +------------------------------+
    | NumPoints(GeomFromText(@ls)) |
    +------------------------------+
    |                            3 |
    +------------------------------+
    
  • PointN(ls,N)

    Returns the N-th Point in the Linestring value ls. Points are numbered beginning with 1.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT AsText(PointN(GeomFromText(@ls),2));
    +-------------------------------------+
    | AsText(PointN(GeomFromText(@ls),2)) |
    +-------------------------------------+
    | POINT(2 2)                          |
    +-------------------------------------+
    
  • StartPoint(ls)

    Returns the Point that is the start point of the LineString value ls.

    mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    mysql> SELECT AsText(StartPoint(GeomFromText(@ls)));
    +---------------------------------------+
    | AsText(StartPoint(GeomFromText(@ls))) |
    +---------------------------------------+
    | POINT(1 1)                            |
    +---------------------------------------+
    

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

  • IsRing(ls)

    Returns 1 if the LineString value ls is closed (that is, its StartPoint() and EndPoint() values are the same) and is simple (does not pass through the same point more than once). Returns 0 if ls is not a ring, and –1 if it is NULL.

11.17.5.2.4. MultiLineString Functions

These functions return properties of MultiLineString values.

  • GLength(mls)

    Returns as a double-precision number the length of the MultiLineString value mls. The length of mls is equal to the sum of the lengths of its elements.

    mysql> SET @mls = 'MultiLineString((1 1,2 2,3 3),(4 4,5 5))';
    mysql> SELECT GLength(GeomFromText(@mls));
    +-----------------------------+
    | GLength(GeomFromText(@mls)) |
    +-----------------------------+
    |             4.2426406871193 |
    +-----------------------------+
    

    GLength() is a nonstandard name. It corresponds to the OpenGIS Length() function.

  • IsClosed(mls)

    Returns 1 if the MultiLineString value mls is closed (that is, the StartPoint() and EndPoint() values are the same for each LineString in mls). Returns 0 if mls is not closed, and –1 if it is NULL.

    mysql> SET @mls = 'MultiLineString((1 1,2 2,3 3),(4 4,5 5))';
    mysql> SELECT IsClosed(GeomFromText(@mls));
    +------------------------------+
    | IsClosed(GeomFromText(@mls)) |
    +------------------------------+
    |                            0 |
    +------------------------------+
    
11.17.5.2.5. Polygon Functions

These functions return properties of Polygon values.

  • Area(poly)

    Returns as a double-precision number the area of the Polygon value poly, as measured in its spatial reference system.

    mysql> SET @poly = 'Polygon((0 0,0 3,3 0,0 0),(1 1,1 2,2 1,1 1))';
    mysql> SELECT Area(GeomFromText(@poly));
    +---------------------------+
    | Area(GeomFromText(@poly)) |
    +---------------------------+
    |                         4 |
    +---------------------------+
    
  • ExteriorRing(poly)

    Returns the exterior ring of the Polygon value poly as a LineString.

    mysql> SET @poly =
        -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
    mysql> SELECT AsText(ExteriorRing(GeomFromText(@poly)));
    +-------------------------------------------+
    | AsText(ExteriorRing(GeomFromText(@poly))) |
    +-------------------------------------------+
    | LINESTRING(0 0,0 3,3 3,3 0,0 0)           |
    +-------------------------------------------+
    
  • InteriorRingN(poly,N)

    Returns the N-th interior ring for the Polygon value poly as a LineString. Rings are numbered beginning with 1.

    mysql> SET @poly =
        -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
    mysql> SELECT AsText(InteriorRingN(GeomFromText(@poly),1));
    +----------------------------------------------+
    | AsText(InteriorRingN(GeomFromText(@poly),1)) |
    +----------------------------------------------+
    | LINESTRING(1 1,1 2,2 2,2 1,1 1)              |
    +----------------------------------------------+
    
  • NumInteriorRings(poly)

    Returns the number of interior rings in the Polygon value poly.

    mysql> SET @poly =
        -> 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
    mysql> SELECT NumInteriorRings(GeomFromText(@poly));
    +---------------------------------------+
    | NumInteriorRings(GeomFromText(@poly)) |
    +---------------------------------------+
    |                                     1 |
    +---------------------------------------+
    
11.17.5.2.6. MultiPolygon Functions

These functions return properties of MultiPolygon values.

  • Area(mpoly)

    Returns as a double-precision number the area of the MultiPolygon value mpoly, as measured in its spatial reference system.

    mysql> SET @mpoly =
        -> 'MultiPolygon(((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1)))';
    mysql> SELECT Area(GeomFromText(@mpoly));
    +----------------------------+
    | Area(GeomFromText(@mpoly)) |
    +----------------------------+
    |                          8 |
    +----------------------------+
    

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

  • Centroid(mpoly)

    Returns the mathematical centroid for the MultiPolygon value mpoly as a Point. The result is not guaranteed to be on the MultiPolygon.

  • PointOnSurface(mpoly)

    Returns a Point value that is guaranteed to be on the MultiPolygon value mpoly.

11.17.5.2.7. GeometryCollection Functions

These functions return properties of GeometryCollection values.

  • GeometryN(gc,N)

    Returns the N-th geometry in the GeometryCollection value gc. Geometries are numbered beginning with 1.

    mysql> SET @gc = 'GeometryCollection(Point(1 1),LineString(2 2, 3 3))';
    mysql> SELECT AsText(GeometryN(GeomFromText(@gc),1));
    +----------------------------------------+
    | AsText(GeometryN(GeomFromText(@gc),1)) |
    +----------------------------------------+
    | POINT(1 1)                             |
    +----------------------------------------+
    
  • NumGeometries(gc)

    Returns the number of geometries in the GeometryCollection value gc.

    mysql> SET @gc = 'GeometryCollection(Point(1 1),LineString(2 2, 3 3))';
    mysql> SELECT NumGeometries(GeomFromText(@gc));
    +----------------------------------+
    | NumGeometries(GeomFromText(@gc)) |
    +----------------------------------+
    |                                2 |
    +----------------------------------+
    
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