[postgis-users] Selecting Unique polygons with st_intersects()

[Ben Madin]

G'day all,

I've just realised that in selecting a group of properties by the local government area they reside in, I end up with duplicates = some properties span shire boundaries. I've used st_intersects, because some properties cover boundaries, so I need those that are within and may be partially without the boundary.

I'm sure I'm not the first person to have this problem, but I was wondering if anyone has any insights into the most efficient way to choose a unique listing of property and shire. I was wondering about taking the area of the property still within the local government area, and choosing the row with the biggest value... 

Although I have unique property identifiers, I can't be sure of not getting a ridiculous answer - an erroneously small amount of a property in a shire etc.

My current query looks a bit like :

SELECT DISTINCT ON (pic) lga_name09 as shire, propname as name, pic 
FROM lga l 
JOIN qldproperties q ON st_intersects(q.the_geom, l.gda_geom) AND l.gid in (245,247,252,254,258,259,275,279,289,297);

but I need to do better than using SELECT DISTINCT ON.

cheers

Ben