[postgis-users] Union, but need to exclude any void

[Toby Rainthorpe]

I have a list of territories, say 9 represented by a tic, tac, toe   
board (3 x 3 grid ). A query selects all the outer perimeter (8  
territories), but omits the central block. If i were then to union  
this group the resulting polygon would fill, or include this central  
block, even though I have not exclusively said that that territory has  
been selected.

Is there a way of doing a union, or another method on a set of  
polygons that will result in a polygon being created, but allow for  
the void not to be included.

Thanks for your help,

Toby


SELECT
	GeomUnion(geom) as the_geom,
FROM (
	SELECT (dump(the_geom)).geom FROM territories WHERE  
is_an_outer_territory = 1
)