I have one PostGIS table of polygons like this:

polygon_id | polygon_geom

And another of points like this:

point_id | polygon_id | point_geom

I want to find all the points from the points table that are within a specific polygon from the polygons table. I tried this query:

SELECT points.point_id FROM points, polygons WHERE St_contains((SELECT polygons.polygon_geom FROM polygons WHERE polygons.polygon_id = 2905), points.point_geom) = 't';

I expect 21 point_id's, but instead it gives me the entire polygon table with each line repeated 21 times. How can I get just the 21 rows from the points table? And furthermore, how can I then assign the polygon_id that these points are within to the points.polygon_id column for those 21 rows in the points table?

EDIT: If I use this query to check if one polygon is in another from the polygon table, this query works perfectly. So I feel like I'm close but fundamentally misunderstanding something.

EDIT 2: changed polygon.id column to polygon.polygon_id


Your problem is the Where clause. The FROM points, polygons syntax gives you a Cartesian join between both tables, ie, it is just syntactic sugar for a full outer join. So, you can use ST_Contains to only return those points that are inside a polygon. The array_agg function will give you all polygons that a point is inside, in the case that there might be more than one.

   pts.point_id, array_agg(poly.polygon_id)
    points pts, polygons poly
 WHERE ST_Contains(poly_geom, point_geom)
 GROUP BY pts.point_id;
  • Thanks! This looks like the answer. My table is pretty big so in order to test this quickly, can I plug in a specific polygon_id to just test one polygon first? Where can I plug that in? – sunday_funday Jan 4 at 15:26
  • ` WHERE ST_Contains(poly_geom, point_geom) AND polygon_id = 1`. Make sure you have a spatial index on the polygon table. – John Powell Jan 4 at 15:38

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