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New cartodb & PostgreSQL user here. I have two rows in cartodb table and each row has its own polygon. The two polygons have some overlapping area. How can I create a new polygon of just the overlapping area, preferably via cartodb SQL commands?

overlapping polygons

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Use ST_Intersection

So something like:

SELECT ST_Intersection(p1.the_geom, p2.the_geom) As the_geom
   FROM p1 INNER JOIN p2 ON (ST_Intersects(p1.geom, p2.geom) );

If you know the ids or some other delimiting factor you could do this:

    SELECT ST_Intersection(p1.the_geom, p2.the_geom) As the_geom
   FROM p1 CROSS JOIN p2  WHERE p1.cartodb_id = 1 and p2.cartodb_id = 2;
  • The CROSS JOIN on rows in a single table did not work, but I was able to use the first method by moving the two rows to two separate tables and using: SELECT ST_Intersection(table1.the_geom, table2.the_geom) As the_geom FROM table1 INNER JOIN table2 ON ST_Intersects(table1.the_geom, table2.the_geom) The results came back in a different format that required running a find and replace and placing square brackets around each coordinate and commas in between each coordinate before being able to save as a polygon into cartoDB. THANK YOU. – Alex Sep 5 '15 at 3:13

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