[postgis-users] Intersection of three polygons

[Saranya Kuniyil]

Hi all,
I am new to postgis and I need to find intersection of three polygons.
In my table named "mytable" contains large no of polygon coordinates and
grouped into three categories by adding a field called "filename".I
labelled the category names as part1, part2, part3 in filename . Now I need
to find all polygon which spacially intersect in three categories.
I write my code as:
 SELECT ST_Intersection(a.geom, b.geom) as intersect_ab
FROM mytable a INNER JOIN mytable b ON ST_Intersects(a.geom,b.geom) INNER
JOIN mytable c ON ST_Intersects(a.geom,c.geom)
WHERE ST_Overlaps(a.geom, b.geom) AND ST_Overlaps(b.geom, c.geom) AND
ST_Overlaps(a.geom, c.geom) AND ST_isvalid(a.geom)='t' AND
ST_isvalid(b.geom)='t' AND ST_isvalid(c.geom)='t' AND a.filename = 'part1'
   AND b.filename = 'part2' AND c.filename = 'part3') ;
Is it correct?????
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