[postgis-users] compute "difference" between two roads

[c. m.]

hi all,
i'm using the navteq data set, postgis/postgres and the uml
mapserver. Actually I have created a table with forest streets and
one with side streets. forest streets are a subset of side streets,
i.e. a forest street is also a side street. now I want to
geometrically compute the difference of the two (side - forest
streets) to get an adjacent side streets table (layer)  to be able
to display them properly and independently on the mapserver. as a
result I always get the entire side street and not the difference
between the two areas/lines...unfortunately it doesn't work and I
can't understand why.

select s.*, difference(s.wkb_geometry, f.intersection_geom) as diff_geom 
into test_side_streets
from sidestreets as s, foreststreets as f
where s.feature_id = f.feature_id
--needed to test on a particular street
--where s.street_name LIKE 'KIENASTENWIES%'

I would be grateful if somebody could give me a hint. thanks a lot.

cheers,
cesare