STIsSimple (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
SQL database in Microsoft Fabric
Returns 1 if a geometry instance is simple, as defined by the Open Geospatial Consortium (OGC). Returns 0 if a geometry instance is not simple.
Syntax
.STIsSimple ( )
Return Types
SQL Server return type: bit
CLR return type: SqlBoolean
Remarks
To be simple, a geometry instance must meet all of the following requirements:
Each figure of the instance must not intersect itself, except at its endpoints.
No two figures of the instance can intersect each other at a point that is not in both of their boundaries.
Examples
The following example creates a nonsimple LineString instance that intersects itself and uses STIsSimple() to test whether the LineString is simple.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('LINESTRING(0 0, 2 2, 0 2, 2 0)', 0);
SELECT @g.STIsSimple();