ST_NumInteriorRings

Returns the count of interior rings in a Polygon. This function calculates the total number of inner holes within the polygon.

Syntax

ST_NumInteriorRings(poly)
NumInteriorRings(poly)

Description

Returns an integer containing the number of interior rings in the Polygon value poly.

Note that according the OpenGIS standard, a POLYGON should have exactly one ExteriorRing and all other rings should lie within that ExteriorRing and thus be the InteriorRings. Practically, however, some systems, including MariaDB's, permit polygons to have several 'ExteriorRings'. In the case of there being multiple, non-overlapping exterior rings ST_NumInteriorRings() will return 1.

ST_NumInteriorRings() and NumInteriorRings() are synonyms.

Examples

SET @poly = 'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';

SELECT NumInteriorRings(GeomFromText(@poly));
+---------------------------------------+
| NumInteriorRings(GeomFromText(@poly)) |
+---------------------------------------+
|                                     1 |
+---------------------------------------+

Non-overlapping 'polygon':

SELECT ST_NumInteriorRings(ST_PolyFromText('POLYGON((0 0,10 0,10 10,0 10,0 0),
  (-1 -1,-5 -1,-5 -5,-1 -5,-1 -1))')) AS NumInteriorRings;
+------------------+
| NumInteriorRings |
+------------------+
|                1 |
+------------------+

_{This page is licensed: GPLv2, originally from fill_help_tables.sql}