STUnion (geography Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns an object that represents the union of a geography instance with another geography instance.
Syntax
.STUnion ( other_geography )
Arguments
other_geography
Is another geography instance to form a union with the instance on which STUnion() 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.
SQL Server supports spatial instances that are larger than a hemisphere. In SQL Server, the set of possible results returned 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.
This method is not precise.
Examples
A. Computing the union of two polygons
The following example uses STUnion() to compute the union 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.STUnion(@h).ToString();
B. Producing a FullGlobe result
The following example produces a FullGlobe when STUnion() combines two Polygon instances.
DECLARE @g geography = 'POLYGON ((-122.358 47.653, -122.358 47.658,-122.348 47.658, -122.348 47.649, -122.358 47.653))';
DECLARE @h geography = 'POLYGON ((-122.358 47.653, -122.348 47.649, -122.348 47.658, -122.358 47.658, -122.358 47.653))';
SELECT @g.STUnion(@h).ToString();
C. Producing a triagonal hole from a union of a CurvePolygon and a triagonal hole.
The following example produces a triagonal hole from the union of a CurvePolygon with a Polygon instance.
DECLARE @g geography = 'POLYGON ((-0.5 0, 0 1, 0.5 0.5, -0.5 0))';
DECLARE @h geography = 'CURVEPOLYGON(COMPOUNDCURVE(CIRCULARSTRING(0 0, 0.7 0.7, 0 1), (0 1, 0 0)))';
SELECT @g.STUnion(@h).ToString();