STPointN (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns a specified point in a geometry instance.
Syntax
.STPointN ( expression )
Arguments
expression
Is an int expression between 1 and the number of points in the geometry instance.
Return Types
SQL Server return type: geometry
CLR return type: SqlGeometry
Open Geospatial Consortium (OGC) type: Point
Remarks
If a geometry instance is user created, STPointN() returns the point specified by expression by ordering the points in the order in which they were originally input.
If a geometry instance was constructed by the system, STPointN() returns the point specified by expression by ordering all the points in the same order they would be output: first by geometry, then by ring within the geometry (if appropriate), and then by point within the ring. This order is deterministic.
If this method is called with a value less than 1, it throws an ArgumentOutOfRangeException.
If this method is called with a value greater than the number of points in the instance, it returns null.
Examples
The following example creates a LineString instance and uses STPointN() to retrieve the second point in the description of the instance.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('LINESTRING(0 0, 2 2, 1 0)', 0);
SELECT @g.STPointN(2).ToString();