STExteriorRing (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns the exterior ring of a geometry instance that is a polygon.
Syntax
.STExteriorRing ( )
Return Types
SQL Server return type: geometry
CLR return type: SqlGeometry
Open Geospatial Consortium (OGC) type: LineString
Remarks
This method returns null if the geometry instance is not a polygon.
Examples
The following example creates a Polygon instance and uses STExteriorRing() to return the exterior ring of the polygon as a LineString.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('POLYGON((0 0, 3 0, 3 3, 0 3, 0 0),(2 2, 2 1, 1 1, 1 2, 2 2))', 0);
SELECT @g.STExteriorRing().ToString();