MySQL:

ST_INTERIORRINGN

ST_InteriorRingN(poly, N)

Returns the N-th interior ring for the Polygon value poly as a
LineString. Rings are numbered beginning with 1.

ST_InteriorRingN() handles its arguments as described in the
introduction to this section.

URL: https://dev.mysql.com/doc/refman/8.0/en/gis-polygon-property-functions.html

Example

mysql> SET @poly =
'Polygon((0 0,0 3,3 3,3 0,0 0),(1 1,1 2,2 2,2 1,1 1))';
mysql> SELECT ST_AsText(ST_InteriorRingN(ST_GeomFromText(@poly),1));
+-------------------------------------------------------+
| ST_AsText(ST_InteriorRingN(ST_GeomFromText(@poly),1)) |
+-------------------------------------------------------+
| LINESTRING(1 1,1 2,2 2,2 1,1 1) |
+-------------------------------------------------------+