STDifference (geography Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
SQL database in Microsoft Fabric
Returns an object that represents the point set from one geography instance that lies outside another geography instance.
Syntax
.STDifference ( other_geography )
Arguments
other_geography
Is another geography instance indicating which points to remove from the instance on which STDifference() is being invoked.
Return Types
SQL Server return type: geography
CLR return type: SqlGeography
Exceptions
This method throws an ArgumentException if the instance contains an antipodal edge.
Remarks
This method always returns null if the spatial reference identifiers (SRIDs) of the geography instances do not match.
In SQL Server, the set of possible results returned on the server has been extended to FullGlobe instances. SQL Server supports spatial instances that are larger than a hemisphere. The result may contain circular arc segments only if the input instances contain circular arc segments. This method is not precise.
Examples
A. Computing the difference between two geography instances
The following example uses STDifference() to compute the difference between two geography instances.
DECLARE @g geography;
DECLARE @h 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);
SET @h = geography::STGeomFromText('LINESTRING(-122.360 47.656, -122.343 47.656)', 4326);
SELECT @g.STDifference(@h).ToString();
B. Using a FullGlobe with STDifference()
The following example uses FullGlobe instance. The first result is an empty GeometryCollection and the second result is a Polygon instance. STDifference() returns an empty GeometryCollection when a FullGlobe instance is the parameter. Every point in an invoking geography instance is contained in a FullGlobe instance.
DECLARE @g geography = 'POLYGON((-122.358 47.653, -122.348 47.649, -122.348 47.658, -122.358 47.658, -122.358 47.653))';
DECLARE @h geography = 'FULLGLOBE';
SELECT @g.STDifference(@h).ToString(),
@h.STDifference(@g).ToString();