STSymDifference (geography Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
SQL database in Microsoft Fabric
Returns an object that represents all points that are either in one geography instance or another geography instance, but not those points that lie in both instances.
Syntax
.STSymDifference ( other_geography )
Arguments
other_geography
Is another geography instance in addition to the instance on which STSymDistance() is being invoked.
Return Types
SQL Server return type: geography
CLR return type: SqlGeography
Remarks
This method always returns null if the spatial reference identifiers (SRIDs) of the geography instances do not match.
SQL Server supports spatial instances that are larger than a hemisphere. In SQL Server, the set of possible results on the server has been extended to FullGlobe instances.
The result may contain circular arc segments only if the input instances contain circular arc segments.
Examples
A. Computing the symmetric difference of two polygons
The following example uses STSymDifference() to compute the symmetric difference of two Polygon 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('POLYGON((-122.351 47.656, -122.341 47.656, -122.341 47.661, -122.351 47.661, -122.351 47.656))', 4326);
SELECT @g.STSymDifference(@h).ToString();
B. Computing the symmetric difference with FullGlobe
The following example compares the symmetric difference of a Polygon with FullGlobe.
DECLARE @g geography = 'POLYGON((-122.358 47.653, -122.348 47.649, -122.348 47.658, -122.358 47.658, -122.358 47.653))';
SELECT @g.STSymDifference('FULLGLOBE').ToString();