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12.16.7.3 LineString and MultiLineString Property Functions

Inhaltsverzeichnis

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.

Some functions in this section also work for MultiLineString values.

Unless otherwise specified, functions in this section handle their arguments as follows:

  • If any argument is NULL or any geometry argument is an empty geometry, the return value is NULL.

  • If any geometry argument is not a syntactically well-formed geometry, an ER_GIS_INVALID_DATA error occurs.

  • If any geometry argument has an SRID value that refers to an undefined spatial reference system (SRS), an ER_SRS_NOT_FOUND error occurs.

  • Otherwise, the return value is non-NULL.

These functions are available for obtaining linestring properties:

  • ST_EndPoint(ls)

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

    ST_EndPoint() handles its arguments as described in the introduction to this section.

    1. mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    2. mysql> SELECT ST_AsText(ST_EndPoint(ST_GeomFromText(@ls)));
    3. +----------------------------------------------+
    4. | ST_AsText(ST_EndPoint(ST_GeomFromText(@ls))) |
    5. +----------------------------------------------+
    6. | POINT(3 3)                                   |
    7. +----------------------------------------------+

  • ST_IsClosed(ls)

    For a LineString value ls, ST_IsClosed() returns 1 if ls is closed (that is, its ST_StartPoint() and ST_EndPoint() values are the same).

    For a MultiLineString value ls, ST_IsClosed() returns 1 if ls is closed (that is, the ST_StartPoint() and ST_EndPoint() values are the same for each LineString in ls).

    ST_IsClosed() returns 0 if ls is not closed, and NULL if ls is NULL.

    ST_IsClosed() handles its arguments as described in the introduction to this section, with this exception:

    1. mysql> SET @ls1 = 'LineString(1 1,2 2,3 3,2 2)';
    2. mysql> SET @ls2 = 'LineString(1 1,2 2,3 3,1 1)';
    3.  
    4. mysql> SELECT ST_IsClosed(ST_GeomFromText(@ls1));
    5. +------------------------------------+
    6. | ST_IsClosed(ST_GeomFromText(@ls1)) |
    7. +------------------------------------+
    8. |                                  0 |
    9. +------------------------------------+
    10.  
    11. mysql> SELECT ST_IsClosed(ST_GeomFromText(@ls2));
    12. +------------------------------------+
    13. | ST_IsClosed(ST_GeomFromText(@ls2)) |
    14. +------------------------------------+
    15. |                                  1 |
    16. +------------------------------------+
    17.  
    18. mysql> SET @ls3 = 'MultiLineString((1 1,2 2,3 3),(4 4,5 5))';
    19.  
    20. mysql> SELECT ST_IsClosed(ST_GeomFromText(@ls3));
    21. +------------------------------------+
    22. | ST_IsClosed(ST_GeomFromText(@ls3)) |
    23. +------------------------------------+
    24. |                                  0 |
    25. +------------------------------------+

  • ST_Length(ls [, unit])

    Returns a double-precision number indicating the length of the LineString or MultiLineString value ls in its associated spatial reference system. The length of a MultiLineString value is equal to the sum of the lengths of its elements.

    ST_Length() computes a result as follows:

    • If the geometry is a valid LineString in a Cartesian SRS, the return value is the Cartesian length of the geometry.

    • If the geometry is a valid MultiLineString in a Cartesian SRS, the return value is the sum of the Cartesian lengths of its elements.

    • If the geometry is a valid LineString in a geographic SRS, the return value is the geodetic length of the geometry in that SRS, in meters.

    • If the geometry is a valid MultiLineString in a geographic SRS, the return value is the sum of the geodetic lengths of its elements in that SRS, in meters.

    ST_Length() handles its arguments as described in the introduction to this section, with these exceptions:

    • If the geometry is not a LineString or MultiLineString, the return value is NULL.

    • If the geometry is geometrically invalid, either the result is an undefined length (that is, it can be any number), or an error occurs.

    • If the length computation result is +inf, an ER_DATA_OUT_OF_RANGE error occurs.

    • If the geometry has a geographic SRS with a longitude or latitude that is out of range, an error occurs:

      Ranges shown are in degrees. The exact range limits deviate slightly due to floating-point arithmetic.

    As of MySQL 8.0.16, ST_Length() permits an optional unit argument that specifies the linear unit for the returned length value. These rules apply:

    • If a unit is specified but not supported by MySQL, an ER_UNIT_NOT_FOUND error occurs.

    • If a supported linear unit is specified and the SRID is 0, an ER_GEOMETRY_IN_UNKNOWN_LENGTH_UNIT error occurs.

    • If a supported linear unit is specified and the SRID is not 0, the result is in that unit.

    • If a unit is not specified, the result is in the unit of the SRS of the geometries, whether Cartesian or geographic. Currently, all MySQL SRSs are expressed in meters.

    A unit is supported if it is found in the INFORMATION_SCHEMA ST_UNITS_OF_MEASURE table. See Section 25.29, “The INFORMATION_SCHEMA ST_UNITS_OF_MEASURE Table”.

    1. mysql> SET @ls = ST_GeomFromText('LineString(1 1,2 2,3 3)');
    2. mysql> SELECT ST_Length(@ls);
    3. +--------------------+
    4. | ST_Length(@ls)     |
    5. +--------------------+
    6. | 2.8284271247461903 |
    7. +--------------------+
    8.  
    9. mysql> SET @mls = ST_GeomFromText('MultiLineString((1 1,2 2,3 3),(4 4,5 5))');
    10. mysql> SELECT ST_Length(@mls);
    11. +-------------------+
    12. | ST_Length(@mls)   |
    13. +-------------------+
    14. | 4.242640687119286 |
    15. +-------------------+
    16.  
    17. mysql> SET @ls = ST_GeomFromText('LineString(1 1,2 2,3 3)', 4326);
    18. mysql> SELECT ST_Length(@ls);
    19. +-------------------+
    20. | ST_Length(@ls)    |
    21. +-------------------+
    22. | 313701.9623204328 |
    23. +-------------------+
    24. mysql> SELECT ST_Length(@ls, 'metre');
    25. +-------------------------+
    26. | ST_Length(@ls, 'metre') |
    27. +-------------------------+
    28. |       313701.9623204328 |
    29. +-------------------------+
    30. mysql> SELECT ST_Length(@ls, 'foot');
    31. +------------------------+
    32. | ST_Length(@ls, 'foot') |
    33. +------------------------+
    34. |     1029205.9131247795 |
    35. +------------------------+

  • ST_NumPoints(ls)

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

    ST_NumPoints() handles its arguments as described in the introduction to this section.

    1. mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    2. mysql> SELECT ST_NumPoints(ST_GeomFromText(@ls));
    3. +------------------------------------+
    4. | ST_NumPoints(ST_GeomFromText(@ls)) |
    5. +------------------------------------+
    6. |                                  3 |
    7. +------------------------------------+

  • ST_PointN(ls, N)

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

    ST_PointN() handles its arguments as described in the introduction to this section.

    1. mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    2. mysql> SELECT ST_AsText(ST_PointN(ST_GeomFromText(@ls),2));
    3. +----------------------------------------------+
    4. | ST_AsText(ST_PointN(ST_GeomFromText(@ls),2)) |
    5. +----------------------------------------------+
    6. | POINT(2 2)                                   |
    7. +----------------------------------------------+

  • ST_StartPoint(ls)

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

    ST_StartPoint() handles its arguments as described in the introduction to this section.

    1. mysql> SET @ls = 'LineString(1 1,2 2,3 3)';
    2. mysql> SELECT ST_AsText(ST_StartPoint(ST_GeomFromText(@ls)));
    3. +------------------------------------------------+
    4. | ST_AsText(ST_StartPoint(ST_GeomFromText(@ls))) |
    5. +------------------------------------------------+
    6. | POINT(1 1)                                     |
    7. +------------------------------------------------+

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Dokument erstellt 26/06/2006, zuletzt geändert 26/10/2018
Quelle des gedruckten Dokuments:https://www.gaudry.be/de/mysql-rf-gis-linestring-property-functions.html

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