MySQL:

ST_EXTERIORRING

ST_ExteriorRing(poly)

Returns the exterior ring of the Polygon value poly as a LineString.

ST_ExteriorRing() 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_ExteriorRing(ST_GeomFromText(@poly)));
+----------------------------------------------------+
| ST_AsText(ST_ExteriorRing(ST_GeomFromText(@poly))) |
+----------------------------------------------------+
| LINESTRING(0 0,0 3,3 3,3 0,0 0) |
+----------------------------------------------------+