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 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 |
+------------------+

URL: https://mariadb.com/kb/en/st_numinteriorrings/