MySQL 4.1 introduces spatial extensions to allow the
generation, storage, and analysis of geographic features.
Currently, these features are available for MyISAM tables only.
This chapter covers the following topics:
The basis of these spatial extensions in the OpenGIS geometry model
MySQL implements spatial extensions following the specification of
the Open GIS Consortium (OGC). This is an international consortium
of more than 250 companies, agencies, and universities participating
in the development of publicly available conceptual solutions that can be
useful with all kinds of applications that manage spatial data.
The OGC maintains a Web site at http://www.opengis.org/.
In 1997, the Open GIS Consortium published the
OpenGIS (R) Simple Features Specifications For SQL, a document that
proposes several conceptual ways for extending an SQL RDBMS to support spatial
data. This specification is available from the Open GIS Web site at
http://www.opengis.org/docs/99-049.pdf.
It contains additional information relevant to this chapter.
MySQL implements a subset of the SQL with Geometry Types
environment proposed by OGC.
This term refers to an SQL environment that has been extended with a
set of geometry types. A geometry-valued SQL column is implemented as
a column that has a geometry type. The specifications describe a set of SQL
geometry types, as well as functions on those types to create and
analyze geometry values.
A geographic feature is anything in the world that has a location.
A feature can be:
An entity. For example, a mountain, a pond, a city.
A space. For example, a postcode area, the tropics.
A definable location. For example, a crossroad,
as a particular place where two streets intersect.
You can also find documents that use the term geospatial feature to
refer to geographic features.
Geometry is another word that denotes a geographic feature.
Originally the word geometry meant measurement of the earth.
Another meaning comes from cartography, referring to the geometric
features that cartographers use to map the world.
This chapter uses all of these terms synonymously:
geographic feature, geospatial feature,
feature, or geometry.
The term most commonly used here is geometry.
Let's define a geometry as a point or an aggregate of
points representing anything in the world that has a location.
The set of geometry types proposed by OGC's SQL with Geometry Types
environment is based on the OpenGIS Geometry Model. In this model,
each geometric object has the following general properties:
It is associated with a Spatial Reference System, which describes the
coordinate space in which the object is defined.
The geometry classes define a hierarchy as follows:
Geometry (non-instantiable)
Point (instantiable)
Curve (non-instantiable)
LineString (instantiable)
Line
LinearRing
Surface (non-instantiable)
Polygon (instantiable)
GeometryCollection (instantiable)
MultiPoint (instantiable)
MultiCurve (non-instantiable)
MultiLineString (instantiable)
MultiSurface (non-instantiable)
MultiPolygon (instantiable)
It is not possible to create objects in non-instantiable classes.
It is possible to create objects in instantiable classes.
All classes have properties, and instantiable classes may also
have assertions (rules that define valid class instances).
Geometry is the base class. It's an abstract class.
The instantiable subclasses of Geometry are restricted to zero-, one-,
and two-dimensional geometric objects that exist in
two-dimensional coordinate space. All instantiable geometry classes are
defined so that valid instances of a geometry class are topologically closed
(that is, all defined geometries include their boundary).
The base Geometry class has subclasses for Point,
Curve, Surface, and GeometryCollection:
Point represents zero-dimensional objects.
Curve represents one-dimensional objects, and has subclass
LineString, with sub-subclasses Line and LinearRing.
Surface is designed for two-dimensional objects and
has subclass Polygon.
GeometryCollection
has specialized zero-, one-, and two-dimensional collection classes named
MultiPoint, MultiLineString, and MultiPolygon
for modeling geometries corresponding to collections of
Points, LineStrings, and Polygons, respectively.
MultiCurve and MultiSurface are introduced as abstract superclasses
that generalize the collection interfaces to handle Curves and Surfaces.
Geometry, Curve, Surface, MultiCurve,
and MultiSurface are defined as non-instantiable classes.
They define a common set of methods for their subclasses and
are included for extensibility.
Point, LineString, Polygon, GeometryCollection,
MultiPoint, MultiLineString, and
MultiPolygon are instantiable classes.
Geometry is the root class of the hierarchy. It is a
non-instantiable class but has a number of properties that are common to
all geometry values created from any of the Geometry subclasses.
These properties are described in the following list. (Particular
subclasses have their own specific properties, described later.)
Geometry Properties
A geometry value has the following properties:
Its type.
Each geometry belongs to one of the instantiable classes in the hierarchy.
Its SRID, or Spatial Reference Identifier. This value identifies
the geometry's associated Spatial Reference System that describes the
coordinate space in which the geometry object is defined.
In MySQL, the SRID value is just an integer associated with the geometry
value. All calculations are done assuming Euclidean (planar) geometry.
Its coordinates in its Spatial Reference System,
represented as double-precision (eight-byte) numbers. All non-empty geometries
include at least one pair of (X,Y) coordinates. Empty geometries contain
no coordinates.
Coordinates are related to the SRID.
For example, in different coordinate systems, the distance between two objects
may differ even when objects have the same coordinates, because the distance
on the planar coordinate system and the distance on the geocentric
system (coordinates on the Earth's surface) are different things.
Its interior, boundary, and exterior.
Every geometry occupies some position in space. The exterior of
a geometry is all space not occupied by the geometry. The interior
is the space occupied by the geometry. The boundary is the
interface between the geometry's interior and exterior.
Its MBR (Minimum Bounding Rectangle), or Envelope.
This is the bounding geometry, formed by the minimum and maximum (X,Y)
coordinates:
Whether the value is simple or non-simple.
Geometry values of types (LineString, MultiPoint,
MultiLineString)
are either simple or non-simple. Each type determines its own assertions
for being simple or non-simple.
Whether the value is closed or not closed.
Geometry values of types (LineString, MultiString) are
either closed
or not closed. Each type determines its own assertions for being closed
or not closed.
Whether the value is empty or non-empty
A geometry is empty if it does not have any points.
Exterior, interior, and boundary of an empty geometry
are not defined (that is, they are represented by a NULL value).
An empty geometry is defined to be always simple and has an area of 0.
Its dimension. A geometry can have a dimension of -1, 0, 1,
or 2:
-1 for an empty geometry.
0 for a geometry with no length and no area.
1 for a geometry with non-zero length and zero area.
2 for a geometry with non-zero area.
Point objects have a dimension of zero. LineString
objects have a dimension of 1. Polygon objects have a
dimension of 2. The dimensions of MultiPoint,
MultiLineString, and MultiPolygon objects are the
same as the dimensions of the elements they consist of.
A Curve is a one-dimensional geometry, usually represented by a sequence
of points. Particular subclasses of Curve define the type of
interpolation between points. Curve is a non-instantiable class.
Curve Properties
A Curve has the coordinates of its points.
A Curve is defined as a one-dimensional geometry.
A Curve is simple if it does not pass through the same point twice.
A Curve is closed if its start point is equal to its end point.
The boundary of a closed Curve is empty.
The boundary of a non-closed Curve consists of its two end points.
A Curve that is simple and closed is a LinearRing.
A Surface is a two-dimensional geometry. It is a non-instantiable
class. Its only instantiable subclass is Polygon.
Surface Properties
A Surface is defined as a two-dimensional geometry.
The OpenGIS specification defines a simple Surface as a geometry that
consists of a single ``patch'' that is associated with a single exterior
boundary and zero or more interior boundaries.
The boundary of a simple Surface is the set of closed curves
corresponding to its exterior and interior boundaries.
A Polygon is a planar Surface representing a multisided
geometry. It is defined by a single exterior boundary and zero or more
interior boundaries, where
each interior boundary defines a hole in the Polygon.
Polygon Examples
On a region map, Polygon objects could represent forests, districts, an
so on.
Polygon Assertions
The boundary of a Polygon consists of a set of LinearRing objects
(that is, LineString objects that are both simple and closed) that make up its
exterior and interior boundaries.
A Polygon has no rings that cross. The rings in the boundary of a
Polygon may intersect at a Point, but only as a tangent.
A Polygon has no lines, spikes, or punctures.
A Polygon has an interior that is a connected point set.
A Polygon may have holes.
The exterior of a Polygon with holes is not connected.
Each hole defines a connected component of the exterior.
The preceding assertions make a Polygon a simple geometry.
A GeometryCollection is a geometry that is a collection of one or more
geometries of any class.
All the elements in a GeometryCollection must be in
the same Spatial Reference System (that is, in the same coordinate system).
There are no other constraints on the elements of a GeometryCollection,
although the
subclasses of GeometryCollection described in the following sections
may restrict membership. Restrictions may be based on:
Element type (for example, a MultiPoint may contain only Point
elements)
Dimension
Constraints on the degree of spatial overlap between elements
A MultiCurve is a geometry collection composed of
Curve elements. MultiCurve is a non-instantiable class.
MultiCurve Properties
A MultiCurve is a one-dimensional geometry.
A MultiCurve is simple if and only if all of its elements are simple;
the only intersections between any two elements occur at points that are
on the boundaries of both elements.
A MultiCurve boundary is obtained by applying the ``mod 2 union
rule'' (also known as the ``odd-even rule''):
A point is in the boundary of a MultiCurve if it is in the
boundaries of an odd number of MultiCurve elements.
A MultiCurve is closed if all of its elements are closed.
The boundary of a closed MultiCurve is always empty.
A MultiSurface is a geometry collection composed of surface elements.
MultiSurface is a non-instantiable class. Its only instantiable
subclass is MultiPolygon.
MultiSurface Assertions
Two MultiSurface surfaces have no interiors that intersect.
Two MultiSurface elements have boundaries that
intersect at most at a finite number of points.
A MultiPolygon is a MultiSurface object composed of
Polygon elements.
MultiPolygon Examples
On a region map, a MultiPolygon could represent a system of lakes.
MultiPolygon Assertions
A MultiPolygon has no two Polygon elements with interiors
that intersect.
A MultiPolygon has no two Polygon elements that cross
(crossing is also forbidden by the previous assertion), or that
touch at an infinite number of points.
A MultiPolygon may not have cut lines, spikes, or punctures. A
MultiPolygon is a regular, closed point set.
A MultiPolygon that has more than one Polygon has an
interior that is not connected. The number of connected components of the interior
of a MultiPolygon is equal to the number of Polygon values in
the MultiPolygon.
MultiPolygon Properties
A MultiPolygon is a two-dimensional geometry.
A MultiPolygon boundary is a set of closed curves
(LineString values) corresponding to the boundaries of its
Polygon elements.
Each Curve in the boundary of the MultiPolygon is in the
boundary of exactly one Polygon element.
Every Curve in the boundary of an Polygon element is
in the boundary of the MultiPolygon.
A Backus-Naur grammar that specifies the formal production rules for writing
WKT values can be found in the OGC specification document referenced near the
beginning of this chapter.
The Well-Known Binary (WKB) representation for geometric values is defined by
the OpenGIS specifications.
It is also defined in the ISO ``SQL/MM Part 3: Spatial'' standard.
WKB is used to exchange geometry data as binary streams represented by
BLOB values containing geometric WKB information.
WKB uses one-byte unsigned integers, four-byte unsigned integers, and eight-byte
double-precision numbers (IEEE 754 format). A byte is eight bits.
For example, a WKB value that corresponds to POINT(1 1) consists of
this sequence of 21 bytes (each represented here by two hex digits):
0101000000000000000000F03F000000000000F03F
The sequence may be broken down into these components:
Byte order : 01
WKB type : 01000000
X : 000000000000F03F
Y : 000000000000F03F
Component representation is as follows:
The byte order may be either 0 or 1 to indicate little-endian or big-endian
storage. The little-endian and big-endian byte orders are also known as
Network Data Representation (NDR) and External Data Representation (XDR),
respectively.
The WKB type is a code that indicates the geometry type. Values from 1 through
7 indicate
Point,
LineString,
Polygon,
MultiPoint,
MultiLineString,
MultiPolygon,
and
GeometryCollection.
A Point value has X and Y coordinates, each represented as a
double-precision value.
WKB values for more complex geometry values are represented by more complex
data structures, as detailed in the OpenGIS specification.
This section describes the data types you can use for representing
spatial data in MySQL, and the functions available for creating and retrieving
spatial values.
MySQL has data types that correspond to OpenGIS classes.
Some of these types hold single geometry values:
GEOMETRY
POINT
LINESTRING
POLYGON
GEOMETRY can store geometry values of any type.
The other single-value types,
POINT and LINESTRING and POLYGON,
restrict their values to a particular geometry type.
The other data types hold collections of values:
MULTIPOINT
MULTILINESTRING
MULTIPOLYGON
GEOMETRYCOLLECTION
GEOMETRYCOLLECTION can store a collection of objects
of any type. The other collection types,
MULTIPOINT and MULTILINESTRING and MULTIPOLYGON and GEOMETRYCOLLECTION,
restrict collection members to those having a particular geometry type.
This section describes how to create spatial values using Well-Known Text
and Well-Known Binary functions that are defined in the OpenGIS standard,
and using MySQL-specific functions.
MySQL provides a number of functions that take as input parameters a
Well-Known Text representation and, optionally, a spatial reference
system identifier (SRID). They return the corresponding geometry.
GeomFromText() accepts a WKT of any geometry type as its first
argument. An implementation also provides type-specific construction
functions for construction of geometry values of each geometry type.
GeomCollFromText(wkt[,srid])
GeometryCollectionFromText(wkt[,srid])
Constructs a GEOMETRYCOLLECTION value using its WKT representation and SRID.
GeomFromText(wkt[,srid])
GeometryFromText(wkt[,srid])
Constructs a geometry value of any type using its WKT representation and SRID.
LineFromText(wkt[,srid])
LineStringFromText(wkt[,srid])
Constructs a LINESTRING value using its WKT representation and SRID.
MLineFromText(wkt[,srid])
MultiLineStringFromText(wkt[,srid])
Constructs a MULTILINESTRING value using its WKT representation and SRID.
MPointFromText(wkt[,srid])
MultiPointFromText(wkt[,srid])
Constructs a MULTIPOINT value using its WKT representation and SRID.
MPolyFromText(wkt[,srid])
MultiPolygonFromText(wkt[,srid])
Constructs a MULTIPOLYGON value using its WKT representation and SRID.
PointFromText(wkt[,srid])
Constructs a POINT value using its WKT representation and SRID.
PolyFromText(wkt[,srid])
PolygonFromText(wkt[,srid])
Constructs a POLYGON value using its WKT representation and SRID.
The OpenGIS specification also describes optional functions for constructing
Polygon or MultiPolygon values based on the WKT representation
of a collection of rings or closed LineString values. These values
may intersect. MySQL does not implement these functions:
BdMPolyFromText(wkt,srid)
Constructs a MultiPolygon value from a
MultiLineString value in WKT format containing
an arbitrary collection of closed LineString values.
BdPolyFromText(wkt,srid)
Constructs a Polygon value from a
MultiLineString value in WKT format containing
an arbitrary collection of closed LineString values.
MySQL provides a number of functions that take as input parameters a
BLOB containing a Well-Known Binary representation
and, optionally, a spatial reference
system identifier (SRID). They return the corresponding geometry.
GeomFromWKT() accepts a WKB of any geometry type as its first
argument. An implementation also provides type-specific construction
functions for construction of geometry values of each geometry type.
GeomCollFromWKB(wkb[,srid])
GeometryCollectionFromWKB(wkt[,srid])
Constructs a GEOMETRYCOLLECTION value using its WKB representation and SRID.
GeomFromWKB(wkb[,srid])
GeometryFromWKB(wkt[,srid])
Constructs a geometry value of any type using its WKB representation and SRID.
LineFromWKB(wkb[,srid])
LineStringFromWKB(wkb[,srid])
Constructs a LINESTRING value using its WKB representation and SRID.
MLineFromWKB(wkb[,srid])
MultiLineStringFromWKB(wkb[,srid])
Constructs a MULTILINESTRING value using its WKB representation and SRID.
MPointFromWKB(wkb[,srid])
MultiPointFromWKB(wkb[,srid])
Constructs a MULTIPOINT value using its WKB representation and SRID.
MPolyFromWKB(wkb[,srid])
MultiPolygonFromWKB(wkb[,srid])
Constructs a MULTIPOLYGON value using its WKB representation and SRID.
PointFromWKB(wkb[,srid])
Constructs a POINT value using its WKB representation and SRID.
PolyFromWKB(wkb[,srid])
PolygonFromWKB(wkb[,srid])
Constructs a POLYGON value using its WKB representation and SRID.
The OpenGIS specification also describes optional functions for constructing
Polygon or MultiPolygon values based on the WKB representation
of a collection of rings or closed LineString values. These values
may intersect. MySQL does not implement these functions:
BdMPolyFromWKB(wkb,srid)
Constructs a MultiPolygon value from a
MultiLineString value in WKB format containing
an arbitrary collection of closed LineString values.
BdPolyFromWKB(wkb,srid)
Constructs a Polygon value from a
MultiLineString value in WKB format containing
an arbitrary collection of closed LineString values.
Note: MySQL does not implement the functions listed in this
section.
MySQL provides a set of useful functions for creating geometry WKB
representations. The functions described in this section are MySQL
extensions to the OpenGIS specifications. The results of these
functions are BLOB values containing WKB representations of geometry
values with no SRID.
The results of these functions can be substituted as the first argument
for any function in the GeomFromWKB() function family.
GeometryCollection(g1,g2,...)
Constructs a WKB GeometryCollection. If any argument is not a
well-formed WKB representation of a geometry, the return value is
NULL.
LineString(pt1,pt2,...)
Constructs a WKB LineString value from a number of WKB Point
arguments. If any argument is not a WKB Point, the return value
is NULL. If the number of Point arguments is less than two,
the return value is NULL.
MultiLineString(ls1,ls2,...)
Constructs a WKB MultiLineString value using WKB LineString
arguments. If any argument is not a WKB LineString, the return
value is NULL.
MultiPoint(pt1,pt2,...)
Constructs a WKB MultiPoint value using WKB Point arguments.
If any argument is not a WKB Point, the return value is NULL.
MultiPolygon(poly1,poly2,...)
Constructs a WKB MultiPolygon value from a set of WKB Polygon
arguments.
If any argument is not a WKB Polygon, the rerurn value is NULL.
Point(x,y)
Constructs a WKB Point using its coordinates.
Polygon(ls1,ls2,...)
Constructs a WKB Polygon value from a number of WKB LineString
arguments. If any argument does not represent the WKB of a LinearRing
(that is, not a closed and simple LineString) the return value
is NULL.
MySQL provides a standard way of creating spatial columns for
geometry types, for example, with CREATE TABLE or ALTER TABLE.
Currently, spatial columns are supported only for MyISAM tables.
Use the CREATE TABLE statement to create a table with a spatial column:
After you have created spatial columns, you can populate them
with spatial data.
Values should be stored in internal geometry format, but you can convert them
to that format from either Well-Known Text (WKT) or Well-Known Binary (WKB)
format. The following examples demonstrate how to insert geometry values into
a table by converting WKT values into internal geometry format.
You can perform the conversion directly in the INSERT statement:
INSERT INTO geom VALUES (GeomFromText('POINT(1 1)'));
SET @g = 'POINT(1 1)';
INSERT INTO geom VALUES (GeomFromText(@g));
Or you can perform the conversion prior to the INSERT:
SET @g = GeomFromText('POINT(1 1)');
INSERT INTO geom VALUES (@g);
The following examples insert more complex geometries into the table:
SET @g = 'LINESTRING(0 0,1 1,2 2)';
INSERT INTO geom VALUES (GeomFromText(@g));
SET @g = 'POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7, 5 5))';
INSERT INTO geom VALUES (GeomFromText(@g));
SET @g =
'GEOMETRYCOLLECTION(POINT(1 1),LINESTRING(0 0,1 1,2 2,3 3,4 4))';
INSERT INTO geom VALUES (GeomFromText(@g));
The preceding examples all use GeomFromText() to create geometry
values. You can also use type-specific functions:
SET @g = 'POINT(1 1)';
INSERT INTO geom VALUES (PointFromText(@g));
SET @g = 'LINESTRING(0 0,1 1,2 2)';
INSERT INTO geom VALUES (LineStringFromText(@g));
SET @g = 'POLYGON((0 0,10 0,10 10,0 10,0 0),(5 5,7 5,7 7,5 7, 5 5))';
INSERT INTO geom VALUES (PolygonFromText(@g));
SET @g =
'GEOMETRYCOLLECTION(POINT(1 1),LINESTRING(0 0,1 1,2 2,3 3,4 4))';
INSERT INTO geom VALUES (GeomCollFromText(@g));
Note that if a client application program wants to use WKB representations
of geometry values, it is responsible for sending correctly formed WKB in
queries to the server. However, there are several ways of satisfying this
requirement. For example:
Inserting a POINT(1 1) value with hex literal syntax:
mysql> INSERT INTO geom VALUES
-> (GeomFromWKB(0x0101000000000000000000F03F000000000000F03F));
An ODBC application can send a WKB representation, binding it to a
placeholder using an argument of BLOB type:
INSERT INTO geom VALUES (GeomFromWKB(?))
Other programming interfaces may support a similar placeholder mechanism.
In a C program, you can escape a binary value using
mysql_real_escape_string() and include the result in a query string
that is sent to the server.
See section 21.2.3.45 mysql_real_escape_string().
After populating spatial columns with values, you are ready to
query and analyze them. MySQL provides a set of functions to
perform various operations on spatial data. These functions can be
grouped into four major categories according to the type of operation
they perform:
Functions that convert geometries between various formats
Functions that provide access to qualitative or quantitative properties of a geometry
Functions that describe relations between two geometries
Functions that create new geometries from existing ones
Spatial analysis functions can be used in many contexts, such as:
Any interactive SQL program, such as mysql or MySQLCC
Application programs written in any language that supports a MySQL client API
Converts a string value from its WKT representation into internal geometry
format and returns the result.
A number of type-specific functions are also supported, such as
PointFromText() and LineFromText(); see
section 18.4.2.1 Creating Geometry Values Using WKT Functions.
GeomFromWKB(wkb[,srid])
Converts a binary value from its WKB representation into internal geometry
format and returns the result.
A number of type-specific functions are also supported, such as
PointFromWKB() and LineFromWKB(); see
section 18.4.2.2 Creating Geometry Values Using WKB 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.
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 18.2.2 Class Geometry.)
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.
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.
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.
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 end point of the LineString value
ls.
Returns 1 if the LineString value ls is closed
(that is, its StartPoint() and EndPoint() values are the same).
Returns 0 if ls is not closed, and -1 if it is NULL.
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.
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.
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.
The OpenGIS specification defines the following functions. Currently,
MySQL does not implement them according to the specification. Those that
are implemented return the same result as the corresponding
MBR-based functions. This includes functions in the following list
other than Distance() and Related().
These functions may be implemented in future releases with full
support for spatial analysis, not just MBR-based support.
The functions operate on two geometry values g1 and g2.
Contains(g1,g2)
Returns 1 or 0 to indicate whether or not g1 completely contains
g2.
Crosses(g1,g2)
Returns 1 if g1 spatially crosses g2.
Returns NULL if g1 is a Polygon or a MultiPolygon,
or if g2 is a Point or a MultiPoint.
Otherwise, returns 0.
The term spatially crosses denotes a spatial relation between two given
geometries that has the following properties:
The two geometries intersect
Their intersection results in a geometry that has
a dimension that is one less than the maximum dimension of the two given
geometries
Their intersection is not equal to either of the two given geometries
Disjoint(g1,g2)
Returns 1 or 0 to indicate whether or not g1 is spatially disjoint
from (does not intersect) g2.
Distance(g1,g2)
Returns as a double-precision number
the shortest distance between any two points in the two geometries.
Equals(g1,g2)
Returns 1 or 0 to indicate whether or not g1 is spatially equal to
g2.
Intersects(g1,g2)
Returns 1 or 0 to indicate whether or not g1 spatially intersects
g2.
Overlaps(g1,g2)
Returns 1 or 0 to indicate whether or not g1 spatially overlaps
g2.
The term spatially overlaps is used if two
geometries intersect and their intersection results in a geometry of the
same dimension but not equal to either of the given geometries.
Related(g1,g2,pattern_matrix)
Returns 1 or 0 to indicate whether or not the spatial relationship specified
by pattern_matrix exists between g1 and g2.
Returns -1 if the arguments are NULL.
The pattern matrix is a string. Its specification will be noted here if this
function is implemented.
Touches(g1,g2)
Returns 1 or 0 to indicate whether or not g1 spatially touches
g2. Two geometries spatially touch if the interiors of
the geometries do not intersect, but the boundary of one of the geometries
intersects either the boundary or the interior of the other.
Within(g1,g2)
Returns 1 or 0 to indicate whether or not g1 is spatially within
g2.
Search operations in non-spatial databases can be optimized
using indexes. This is true for spatial databases as well.
With the help of a great variety of multi-dimensional indexing methods that
have already been designed, it is possible to optimize
spatial searches. The most typical of these are:
Point queries that search for all objects that contain a given point
Region queries that search for all objects that overlap a given region
MySQL uses R-Trees with quadratic splitting to index
spatial columns. A spatial index is built using the MBR of a geometry.
For most geometries, the MBR is a minimum rectangle that
surrounds the geometries. For a horizontal or a vertical
linestring, the MBR is a rectangle degenerated into the linestring.
For a point, the MBR is a rectangle degenerated into the point.
MySQL can create spatial indexes using syntax similar to that for creating
regular indexes, but extended with the SPATIAL keyword.
Spatial columns that are indexed currently must be declared NOT NULL.
The following examples demonstrate how to create spatial indexes.
With CREATE TABLE:
mysql> CREATE TABLE geom (g GEOMETRY NOT NULL, SPATIAL INDEX(g));
With ALTER TABLE:
mysql> ALTER TABLE geom ADD SPATIAL INDEX(g);
With CREATE INDEX:
mysql> CREATE SPATIAL INDEX sp_index ON geom (g);
To drop spatial indexes, use ALTER TABLE or DROP INDEX:
With ALTER TABLE:
mysql> ALTER TABLE geom DROP INDEX g;
With DROP INDEX:
mysql> DROP INDEX sp_index ON geom;
Example: Suppose that a table geom contains more than 32,000 geometries,
which are stored in the column g of type GEOMETRY.
The table also has an AUTO_INCREMENT column fid for storing
object ID values.
mysql> DESCRIBE geom;
+-------+----------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+----------+------+-----+---------+----------------+
| fid | int(11) | | PRI | NULL | auto_increment |
| g | geometry | | | | |
+-------+----------+------+-----+---------+----------------+
2 rows in set (0.00 sec)
mysql> SELECT COUNT(*) FROM geom;
+----------+
| count(*) |
+----------+
| 32376 |
+----------+
1 row in set (0.00 sec)
To add a spatial index on the column g, use this statement:
The optimizer investigates whether available spatial indexes can
be involved in the search for queries that use a function such as
MBRContains() or MBRWithin() in the WHERE clause.
For example, let's say we want to find all objects that are in the
given rectangle:
OpenGIS specifications propose several additional metadata views.
For example, a system view named GEOMETRY_COLUMNS contains a
description of geometry columns, one row for each geometry column
in the database.
The OpenGIS function Length() on LineString and MultiLineString currently should be called in MySQL as GLength()
The problem is that there is an existing SQL function
Length() which calculates the length of string values,
and sometimes it is not possible to distinguish whether the function is
called in a textual or spatial context. We need either to solve this
somehow, or decide on another function name.
