STIsClosed (geometry 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 geometry instance are the same. Returns 1 for geometrycollection types if each contained geometry instance is closed. Returns 0 if the instance is not closed.
Syntax
.STIsClosed ( )
Return Types
SQL Server return type: bit
CLR return type: SqlBoolean
Remarks
This method returns 0 if any figures of a geometry instance are points, or if the instance is empty.
All Polygon instances are considered closed.
Examples
The following example creates a LineString instance and uses STIsClosed() to test if the LineString is closed.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('LINESTRING(0 0, 2 2, 1 0)', 0);
SELECT @g.STIsClosed();