STBoundary (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns the boundary of a geometry instance.
Syntax
.STBoundary ( )
Return Types
SQL Server return type: geometry
CLR return type: SqlGeometry
Remarks
STBoundary() returns an empty GeometryCollection when the endpoints for a LineString, CircularString, or CompoundCurve instance are the same.
Examples
A. Using STBoundary() on a LineString instance with different endpoints
The following example creates a LineString``geometry instance. STBoundary() returns the boundary of the LineString.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('LINESTRING(0 0, 2 2, 0 2, 2 0)', 0);
SELECT @g.STBoundary().ToString();
B. Using STBoundary() on a LineString instance with the same endpoints
The following example creates a valid LineString instance with the same endpoints. STBoundary() returns an empty GeometryCollection.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('LINESTRING(0 0, 2 2, 0 2, -2 2, 0 0)', 0);
SELECT @g.STBoundary().ToString();
C. Using STBoundary() on a CurvePolygon instance
The following example uses STBoundary() on a CurvePolygon instance. STBoundary() returns a CircularString instance.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('CURVEPOLYGON(CIRCULARSTRING(0 0, 2 2, 0 2, -2 2, 0 0))', 0);
SELECT @g.STBoundary().ToString();