SQL:Unión de polígonos

En SQL Server 2012:

SELECT geometry::UnionAggregate(geomcolumn) FROM YourTable;

En SQL Server 2008/R2:

DECLARE @g = geometry::STGeomFromText('GEOMETRYCOLLECTION EMPTY', YourSRID);
SELECT @g = @g.STUnion(geomcolum) FROM YourTable;