SQL:多角形の結合

SQL Server 2012 の場合:

SELECT geometry::UnionAggregate(geomcolumn) FROM YourTable;
  

 SQL Server 2008/R2 の場合:
 
DECLARE @g = geometry::STGeomFromText('GEOMETRYCOLLECTION EMPTY', YourSRID);
SELECT @g = @g.STUnion(geomcolum) FROM YourTable;