STAsBinary (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns the Open Geospatial Consortium (OGC) Well-Known Binary (WKB) representation of a geometry instance.
Syntax
.STAsBinary ( )
Return Types
SQL Server return type: varbinary(max)
CLR return type: SqlBytes
Examples
The following example creates a LineString geometry instance from (0,0) to (2,3) from text. STAsBinary() returns the result in WKB.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('LINESTRING(0 0, 2 3)', 0);
SELECT @g.STAsBinary();