ST_MPolyFromWKB function

The ST_MPolyFromWKB function takes a well-known binary representation of a multipolygon and, optionally, a spatial reference system identifier as input parameters and returns the corresponding multipolygon.

If the given well-known binary representation is null, then null is returned.

The recommended function for achieving the same result is the ST_MultiPolygon function. It is recommended because of its flexibility: ST_MultiPolygon takes additional forms of input as well as the well-known binary representation.

Syntax

Parameters

wkb

A value of type BLOB(2G) that contains the well-known binary representation of the resulting multipolygon.

srs_id

A value of type INTEGER that identifies the spatial reference system for the resulting multipolygon.

If the srs_id parameter is omitted, the spatial reference system with the numeric identifier 0 (zero) is used.

If the specified srs_id does not identify a spatial reference system listed in the catalog view DB2GSE.ST_SPATIAL_REFERENCE_SYSTEMS, then an exception condition is raised (SQLSTATE 38SU1).

Return type

db2gse.ST_MultiPolygon

Example

In the following example, the lines of results have been reformatted for readability. The spacing in your results will vary according to your online display.

This example illustrates how ST_MPolyFromWKB can be used to create a multipolygon from its well-known binary representation. The geometry is a multipolygon in spatial reference system 1. In this example, the multipolygon gets stored with ID = 10 in the GEOMETRY column of the SAMPLE_MPOLYS table, and then the WKB column is updated with its well-known binary representation (using the ST_AsBinary function). Finally, the ST_MPolyFromWKB function is used to return the multipolygon from the WKB column. The X and Y coordinates for this geometry are:

Polygon 1: (1, 72) (4, 79) (5, 76) (1, 72)
Polygon 2: (10, 20) (10, 40) (30, 41) (10, 20)
Polygon 3: (9, 43) (7, 44) (6, 47) (9, 43)

The SAMPLE_MPOLYS table has a GEOMETRY column, where the multipolygon is stored, and a WKB column, where the multipolygon's well-known binary representation is stored.


SET CURRENT FUNCTION PATH = CURRENT FUNCTION PATH, db2gse
CREATE TABLE sample_mpolys (id INTEGER, 
    geometry ST_MultiPolygon, wkb BLOB(32K))

INSERT INTO sample_mpolys
  VALUES (10, ST_MultiPolygon ('multipolygon 
        (( (1 72, 4 79, 5 76, 1 72),
         (10 20, 10 40, 30 41, 10 20),
         (9 43, 7 44, 6 47, 9 43) ))', 1))

UPDATE sample_mpolys AS temporary_correlated
  SET wkb = ST_AsBinary( geometry )
  WHERE id = temporary_correlated.id

In the following SELECT statement, the ST_MPolyFromWKB function is used to retrieve the multipolygon from the WKB column.


SELECT id, CAST( ST_AsText( ST_MPolyFromWKB (wkb) ) 
    AS VARCHAR(320) ) MULTIPOLYGON
  FROM sample_mpolys
  WHERE id = 10

Results:


ID         MULTIPOLYGON
---------- --------------------------------------------------------------------
        10 MULTIPOLYGON ((( 10.00000000 20.00000000, 30.00000000 
              41.00000000, 10.00000000 40.00000000, 10.00000000
              20.00000000)),
                          ( 1.00000000 72.00000000, 5.00000000 
              76.00000000, 4.00000000 79.0000000, 1.00000000 
              72,00000000)),
                           ( 9.00000000 43.00000000, 6.00000000
               47.00000000, 7.00000000 44.00000000, 9.00000000 
               43.00000000 )))