ST_NumPolygons function

The ST_NumPolygons function takes a multipolygon as an input parameter and returns the number of polygons that it contains.

If the specified multipolygon is null or is empty, then null is returned.

Syntax

Parameter

multipolygon: A value of type ST_MultiPolygon that represents the multipolygon for which the number of polygons is returned.

Return type

INTEGER

Example

Multipolygons are stored in the SAMPLE_MPOLYS table. The ST_NumPolygons function determines how many individual geometries are within each multipolygon.


CREATE TABLE sample_mpolys (id INTEGER, geometry ST_MultiPolygon)

INSERT INTO sample_mpolys
  VALUES (1,
          ST_MultiPolygon ('multipolygon (( (3 3, 4 6, 5 3, 3 3),
                                 (8 24, 9 25, 1 28, 8 24),
                                 (13 33, 7 36, 1 40, 10 43, 13 33) ))', 1) )

INSERT INTO sample_polys
  VALUES (2,
          ST_MultiPolygon ('multipolygon empty', 1) )

INSERT INTO sample_polys
  VALUES (3,
          ST_MultiPolygon ('multipolygon (( (3 3, 4 6, 5 3, 3 3),
                                 (13 33, 7 36, 1 40, 10 43, 13 33) ))', 1) )

SELECT id, ST_NumPolygons (geometry) NUM_WITHIN
  FROM sample_mpolys

Results:


ID          NUM_WITHIN
----------- ----------
          1          3
          2          0
          3          2