Hi Gus,
The problem is that your last query is creating the cartesian
product of tables orig1 and orig2. See the URL below for detailed
explanations of what a cartesian product is:
http://www.fluffycat.com/SQL/Cartesian-Joins/
http://www.google.com.br/search?q=cartesian+product&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a
In practice, you are producing the combinations of every feature in
table orig1 with every other feature in orig2. For this reason, you are
adding the area of each feature several times.
To correct that you need to constrain the cartesian product by
creating a proper join. That is, you must say which subset (rows) of the
cartesian product you want returned. You can do this by either using the
JOIN table ON (a.id = b.id) construct or adding the equivalent
constraint to the WHERE clause.
I hope this helps.
Cheers
Ricardo
Gustavo Ces wrote:
Hi all,
I´ve got two tables with geoms. When i try to sum(area) something
strange happens:
select sum(area(the_geom)) from orig1
61301.4750045538
select sum(area(the_geom))from orig2
33996.0705435276
but now...
select sum(area(a.the_geom)) as sum1,sum(area(b.the_geom))as sum2 from
orig1 a,orig2 b
sum1 sum2
11218169.9258333 4963426.29935503
smells like postgresql basic concept unknowed for me.... :(
Gus
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