Re: sum multiple tables gives wrong answer?

On Wed, Jun 2, 2010 at 7:23 AM, Michael Diener  wrote:

> select  sum(flaeche)/10000 as "greens HA"  from green;

> result:

> Wrong Answer with this query
>
> select
>  sum(green.flaeche)/10000 as "greens HA",
>  sum (fairway.flaeche)/10000 as "fairway HA"
>   from green, fairway;

It isn't easy to see but you are effectively joining green to fairway
using a cross project which product a Cartesian product.

you probably wanted this query:

SELECT (select  sum(flaeche)/10000  from green) AS "greens HA",
             (select  sum(flaeche)/10000  from fairway) AS "fairway HA";


However, from what you've shown.  I would wager that your database is
in need of some normalization.  For example you could put both greens
and fair way into a single table like:

CREATE TABLE Lawns AS
SELECT flaech, "green"::VARCHAR AS lawntype
  FROM green
UNION ALL
SELECT flaech, "fairway"::VARCHAR AS lawntype
  FROM fairway;

Then you'd execute the following query:

SELECT lawntype, sum(flaech)/10000 AS  "HA"
  FROM Lawns
GROUP BY lawntype;

-- 
Regards,
Richard Broersma Jr.

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