Show points that are within several overlapped/intersected polygons using ST_Within in PostGIS

Question

I have a separate table for points and polygons. The polygons are overlapped or intersect with each other. The points are within several polygons at the same time. I need to develop an SQL query which should show the points that are in the polygons. If I use an SQL query like this:

SELECT point_table.geom, polygon_table.name FROM point_table
INNER JOIN polygon_table on ST_Within(point_table.geom, polygon_table.geom);

It does not iterate through all polygons, instead it evaluates the point geometry with the innermost polygon and does not evalute the point geometry with other overlapped or intersected polygons. What I need is to evaulate all points with all polygons. Becuase points can be within several polygons at the same time.

For instance, if 10 points are within 3 overlapped polygons, it should return 30 records and each record/row should have the corresponding polygon_table.name in it. And in this situation, the above query is returning 10 records with polygon_table.name from the innermost polygon, ie once it finds the match, it is not evaluating the points against other polygons.

Any help on this matter appreciated.

Thanks.

Answer 1

Your query looks just fine. The error is most probably somewhere else. To make my point clearer, I will add an example showing two polygons and four points, where three points are within the inner polygon and one point is within the outer one:

WITH pol(id,geom) AS (
  VALUES (1,'SRID=4326;POLYGON((-4.53 54.21000000000001,-4.51 54.21000000000001,-4.51 54.19,-4.53 54.19,-4.53 54.21000000000001))'::GEOMETRY),
         (2,'SRID=4326;POLYGON((-4.537478046647734 54.213934294785226,-4.502287464372343 54.213934294785226,-4.502287464372343 54.18506473763151,-4.537478046647734 54.18506473763151,-4.537478046647734 54.213934294785226))'::GEOMETRY)
), poi(id,geom) AS (
  VALUES (1,'SRID=4326;POINT(-4.524700698880288 54.20613765421879)'::GEOMETRY),
         (2,'SRID=4326;POINT(-4.517588984076674 54.20219317274976)'::GEOMETRY),
         (3,'SRID=4326;POINT(-4.524762007263298 54.19627574559112)'::GEOMETRY),
         (4,'SRID=4326;POINT(-4.507098941702792 54.211414844419664)'::GEOMETRY))
SELECT poi.id AS point_id, pol.id AS polygon_id FROM poi
JOIN pol ON ST_Within(poi.geom,pol.geom);

 point_id | polygon_id 
----------+------------
        1 |          1
        1 |          2
        2 |          1
        2 |          2
        3 |          1
        3 |          2
        4 |          2

In the resultset you can see that three points are joined with all polygons and one point is joined with a single one, as you can also see in the image above.

Try it yourself: db<>fiddle

If it still does not work, provide some sample data and we can test it.