STIsClosed (geography Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
SQL database in Microsoft Fabric
Returns 1 if the start and end points of the given geography instance are the same. Returns 1 for geography collection types if each contained geography instance is closed. Returns 0 if the instance is not closed.
This geography data type method supports FullGlobe instances or spatial instances that are larger than a hemisphere.
Syntax
.STIsClosed ( )
Return Types
SQL Server return type: bit
CLR return type: SqlBoolean
Remarks
This method returns 0 if any figures of a geography instance are points, or if the instance is empty.
This method returns true if a FullGlobe instance is a Polygon or other type of instance.
All Polygon instances are considered closed.
Examples
The following example creates a Polygon instance and uses STIsClosed() to test if the Polygon is closed.
DECLARE @g geography;
SET @g = geography::STGeomFromText('POLYGON((-122.358 47.653, -122.348 47.649, -122.348 47.658, -122.358 47.658, -122.358 47.653))', 4326);
SELECT @g.STIsClosed();