Geographic Information Systems Asked by ferus89 on July 12, 2021
I’m working with a spatial PostgreSQL database that contains species records, there is a point layer 'spp_points'
, each point has a "spp_ID"
and "spp_score"
field. I also have a polygon layer 'field_poly'
.
I need to work out the sum of unique "spp_id"
scores only within each polygon, so multiple records of the same species should only be counted once.
I have tried using the following query but this calculates the total of all species, so multiple instances of the same species are included in the sum:
SELECT site.gid AS field,
SUM(spp.total) AS field_score,
site.geom
FROM (SELECT DISTINCT spp_id,
spp_score AS total,
geom
FROM spp_points) AS spp
JOIN field_poly AS site ON ST_Within(spp.geom, site.geom)
GROUP BY site.gid
I’m not sure where I need to go from here.
Get the AVG
of the spp_points.spp_score
in a sub-query, i.e.
SELECT gid,
SUM(spp_scores) AS field_score
FROM (
SELECT ply.gid,
AVG(pnt.spp_score) AS spp_scores
FROM field_poly AS ply
JOIN spp_points AS pnt
ON ST_Intersects(ply.geom, pnt.geom)
GROUP BY
ply.gid, pnt.spp_id
) q
GROUP BY
gid
;
This is semantically equal to
SELECT gid,
SUM(spp_score) AS field_score
FROM (
SELECT DISTINCT ON (ply.gid, pnt.spp_id, pnt.spp_score)
ply.gid,
pnt.spp_id,
pnt.spp_score
FROM field_poly AS ply
JOIN spp_points AS pnt
ON ST_Intersects(ply.geom, pnt.geom)
) q
GROUP BY
gid
;
However, in cases where DISTINCT [ON]
is interchangeable with GROUP BY
, the latter usually outperforms the former.
Correct answer by geozelot on July 12, 2021
From what I understand, you're almost there. You just need to also group by "spp_id"
:
SELECT data.spp_id,
SUM(data.spp_score) AS total_score,
site.gid AS site_id,
site.geom AS site_geom
FROM spp_points as data
INNER JOIN field_poly AS site ON ST_Within(data.geom, site.geom)
GROUP BY site.gid, site.geom, data.spp_id
Answered by robin loche on July 12, 2021
I believe the crux of this problem is in assigning a row_number to all the sightings using the partition on the species ID and the site_gid they are in, and then ordering by the sighting date - which involves using a DATE column - then using only one of each sighting from each polygon.
I've done some mocking up of this using crime and neighborhood data for denver, but re-coded best I could to match your data - my species ID is instead a crime type, and I've used some dummy integer to mock up the spp_score.
Here's the code:
;with cte_data as (
select
f.site_gid
, s.*
, row_number() over (partition by s."spp_ID", f.site_gid order by s.sighting_date desc ) as rownum
from spp_points as s
join field_poly as f on ST_INtersects(s.geom, f.geom)
)
select
d.site_gid
, d."spp_ID"
, sum(d.spp_score)
from cte_data as d
where 1=1 and d.rownum = 1
group by d.site_gid, d."spp_ID"
Which gives this result, which is one species (crime) per field polygon and sums the spp_score (my dummy offense code integer).
I think its almost what you're after, but let me know what you think.
Answered by DPSSpatial_BoycottingGISSE on July 12, 2021
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