ST_NumInteriorRing function

the ST_NumInteriorRing function takes a polygon as an input parameter and returns the number of its interior rings.

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

If the polygon has no interior rings, then 0 (zero) is returned.

Syntax

Parameter

polygon: A value of type ST_Polygon that represents the polygon for which the number of interior rings is returned.

Return type

INTEGER

Example

The following example creates two polygons:

One with two interior rings
One without any interior rings


CREATE TABLE sample_polys (id INTEGER, geometry ST_Polygon)

INSERT INTO sample_polys
  VALUES (1, ST_Polygon('polygon 
        ((40 120, 90 120, 90 150, 40 150, 40 120),
         (50 130, 60 130, 60 140, 50 140, 50 130),
         (70 130, 80 130, 80 140, 70 140, 70 130))' , 0) )

INSERT INTO sample_polys
  VALUES (2, ST_Polygon('polygon ((5 15, 50 15, 50 105, 5 15))' , 0) )

The ST_NumInteriorRing function is used to return the number of rings in the geometries in the table:


SELECT id, ST_NumInteriorRing(geometry) NUM_RINGS 
FROM sample_polys

Results:


ID         NUM_RINGS
---------- ---------------
         1               2
         2               0