This should give you what you're looking for:
SELECT
(SELECT the_geom FROM hyd_usgsstrm WHERE ST_DWithin(p.the_geom,
the_geom, 1) ORDER BY ST_Distance(p.the_geom, the_geom) ASC LIMIT 1), p.*
FROM par_parcels p;
If you need more than the_geom returned from hyd_usgsstrm you can retrieve the
gid in the subquery and then turn the whole query into a derived table and join
back on hyd_usgsstrm for other fields:
SELECT p.*, u.a, u.b, u.the_geom
SELECT
(SELECT gid FROM hyd_usgsstrm WHERE ST_DWithin(p.the_geom,
the_geom, 1) ORDER BY ST_Distance(p.the_geom, the_geom) ASC LIMIT 1) AS u_gid,
p.*
FROM par_parcels p
) p
JOIN hyd_usgsstrm u ON u.gid = p.u_gid;
On Jan 4, 2011, at 8:53 AM, Dan Lyke wrote:
> I have a silly question that I can't wrap my head around doing more
> efficiently (and it may not actually matter).
>
> My example is that I have two tables, tax parcels from the county, and
> the USGS streams center lines.
>
> Say I want to select all parcels within a meter of a stream center line
> (yeah, it's a silly query, as at least one of those "streams" is 25
> meters wide). I can do:
>
> SELECT par_parcels.the_geom FROM par_parcels , hyd_usgsstrm
> WHERE ST_DWithin(par_parcels.the_geom, hyd_usgsstrm.the_geom, 1)
>
> But that, of course, gives me multiple par_parcels.
>
> I can also do:
>
> SELECT DISTINCT(par_parcels.the_geom) FROM par_parcels , hyd_usgsstrm
> WHERE ST_DWithin(par_parcels.the_geom, hyd_usgsstrm.the_geom, 1)
>
> But what I really want is something more like:
>
> SELECT the_geom FROM par_parcels
> WHERE ST_DWithin(the_geom, (SELECT the_geom FROM hyd_usgsstrm), 1);
>
> but that, of course, both doesn't let the query optimizer do anything
> intelligent and gives me:
>
> ERROR: more than one row returned by a subquery used as an expression
>
> What's the right approach for this kind of query?
>
> Thanks!
>
> Dan
>
>
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