[postgis-users] Area of non-intersecting parts of layers

[Cedric Duprez]

Hi all,

I have 2 layers composed of multipolygons in Postgis 2.0. Each layer contains polygons with a name that is sometime common to several polygons.
When I try to find area of intersections between the 2 layers, no problem with the following query:
SELECT l1.name1, l2.name2, SUM(ST_Area(ST_Intersection(l1.geom, l2.geom))) / 10000 AS surface_ha
FROM layer1 l1
INNER JOIN layer2 l2 ON ST_Intersects(l1.geom, l2.geom)
GROUP BY 1, 2
ORDER BY 1, 2;

OK.
But I also try to get the area of my layer1 (grouped by name1) that does not contain elements of my layer2.

I tried with LEFT JOIN, with ST_CONTAINS, without success.

Does anyone have an idea on how to write this query?

Thanks in advance,

Cedric Duprez
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