Thread: sum multiple tables gives wrong answer?

On 2 June 2010 15:23, Michael Diener <m.diener@gomogi.com> wrote:
> Hi,
>
>
>
> I’m new to the list and have the following situation happening "PostgreSQL
> 8.4.1, compiled by Visual C++ build 1400, 32-bit" running windows XP sp3
>
>
>
> I have an SQL problem that I thought was easy to do but gives me always the
> wrong answer.
>
>
>
> 2 Tables with a column called “flaeche” “double precision”, in English
>  “area” and I want to sum up the values for flaeche in each table to give me
> the total area for flaeche in each table.
>
>
>
> Correct answer comes with this sql
>
> select  sum(flaeche)/10000 as "greens HA"  from green;
>
>
>
> result:
>
> greenHA
>
> 1.25358085
>
>
>
> Wrong Answer with this query
>
> select
>
>  sum(green.flaeche)/10000 as "greens HA",
>
>  sum (fairway.flaeche)/10000 as "fairway HA"
>
>   from green, fairway;
>
>
>
> result:
>
> green HA                   fairway HA
>
> 48.8896531                 508.94143659
>
>
>
> Fairway correct answer is  14.96886578 HA
>
> Green correct answer is 1.25358085  HA
>
>
>
> What is going on ??
>
>
>
> Cheers
>
> michael
>
>
>
> Michael Diener
>
> _________________________________________________________________

Could it be because you're effectively using a cartesian join?

Can't you do them separately?  Like:

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

Or if you must have both in the same result:

select (select sum(green.flaeche)/10000 from green) as "greens HA",
(select sum (fairway.flaeche)/10000 from fairway) as "fairway HA";

Regards

Thom

Michael Diener wrote on 02.06.2010 16:23:
> Hi,
>
> I’m new to the list and have the following situation happening
> "PostgreSQL 8.4.1, compiled by Visual C++ build 1400, 32-bit" running
> windows XP sp3
>
> I have an SQL problem that I thought was easy to do but gives me always
> the wrong answer.
>
> 2 Tables with a column called “flaeche” “double precision”, in English
> “area” and I want to sum up the values for flaeche in each table to give
> me the total area for flaeche in each table.
>
> Wrong Answer with this query
>
> select
> sum(green.flaeche)/10000 as "greens HA",
> sum (fairway.flaeche)/10000 as "fairway HA"
> from green, fairway;

You are creating a cartesian product from the two tables.
You need to JOIN them properly, so that rows from green are properly related to the rows from the fairway table

As you didn't provide the table structure, we need to guess:

select sum(green.flaeche)/10000 as "greens HA",
        sum (fairway.flaeche)/10000 as "fairway HA"
FORM green
    JOIN fairway ON green.some_id = fairway.green_id

Thomas

On Wed, Jun 2, 2010 at 7:23 AM, Michael Diener <m.diener@gomogi.com> 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.

Visit the Los Angeles PostgreSQL Users Group (LAPUG)
http://pugs.postgresql.org/lapug

In response to Michael Diener :
> 2 Tables with a column called ?flaeche? ?double precision?, in English  ?area?
> and I want to sum up the values for flaeche in each table to give me the total
> area for flaeche in each table.
>
>
> Wrong Answer with this query
>
> select
>  sum(green.flaeche)/10000 as "greens HA",
>  sum (fairway.flaeche)/10000 as "fairway HA"
>   from green, fairway;
>
> What is going on ??

It's a so called cross-join, every row form the first table crossed with
evvery row from the other table -> wrong result.

Simple example:

test=*# select * from t1;
 id | flaeche
----+---------
  1 |      10
  2 |      20
(2 Zeilen)

Zeit: 0,229 ms
test=*# select * from t2;
 id | flaeche
----+---------
  1 |     100
  2 |     200
(2 Zeilen)

Zeit: 0,182 ms
test=*# select sum(t1.flaeche), sum(t2.flaeche) from t1, t2;
 sum | sum
-----+-----
  60 | 600
(1 Zeile)


It's just this:

test=*# select * from t1, t2;
 id | flaeche | id | flaeche
----+---------+----+---------
  1 |      10 |  1 |     100
  1 |      10 |  2 |     200
  2 |      20 |  1 |     100
  2 |      20 |  2 |     200
(4 Zeilen)




But you are looking for:

test=*# select (select sum(flaeche) from t1) as t1_flaeche, (select
sum(flaeche) from t2);
 t1_flaeche | ?column?
------------+----------
         30 |      300
(1 Zeile)


Regards, Andreas
--
Andreas Kretschmer
Kontakt:  Heynitz: 035242/47150,   D1: 0160/7141639 (mehr: -> Header)
GnuPG: 0x31720C99, 1006 CCB4 A326 1D42 6431  2EB0 389D 1DC2 3172 0C99

----- Original Message -----

From: Michael Diener

To: pgsql-novice@postgresql.org

Cc: pgsql-general@postgresql.org

Sent: Wednesday, June 02, 2010 3:23 PM

Subject: [NOVICE] sum multiple tables gives wrong answer?

Hi,

 

I’m new to the list and have the following situation happening "PostgreSQL 8.4.1, compiled by Visual C++ build 1400, 32-bit" running windows XP sp3

 

I have an SQL problem that I thought was easy to do but gives me always the wrong answer.

 

2 Tables with a column called “flaeche” “double precision”, in English  “area” and I want to sum up the values for flaeche in each table to give me the total area for flaeche in each table.

 

Correct answer comes with this sql

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

 

result:

greenHA

1.25358085

 

Wrong Answer with this query

select

 sum(green.flaeche)/10000 as "greens HA",

 sum (fairway.flaeche)/10000 as "fairway HA"

  from green, fairway;

 

result:

green HA                   fairway HA

48.8896531                 508.94143659

 

Fairway correct answer is  14.96886578 HA

Green correct answer is 1.25358085  HA

 

What is going on ??

 

Cheers

michael

 

Michael Diener

_________________________________________________________________

GOMOGI Mobile Geographics

LAKESIDE PARK B01

9020 KLAGENFURT

 

T: ++043 (0) 676 520 3600

E: m.diener@gomogi.com

W: www.gomogi.com

 

I just want to say thanks to all the great responses it is now working like
a charm!!  I knew I was missing some tid bit of DB SQL knowledge.

Thanks again!

Cheers
michael

-----Original Message-----
From: pgsql-general-owner@postgresql.org
[mailto:pgsql-general-owner@postgresql.org] On Behalf Of Richard Broersma
Sent: Mittwoch, 02. Juni 2010 17:39
To: m.diener@gomogi.com
Cc: pgsql-novice@postgresql.org; pgsql-general@postgresql.org
Subject: Re: [GENERAL] [NOVICE] sum multiple tables gives wrong answer?

On Wed, Jun 2, 2010 at 7:23 AM, Michael Diener <m.diener@gomogi.com> 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.

Visit the Los Angeles PostgreSQL Users Group (LAPUG)
http://pugs.postgresql.org/lapug

--
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I replied just to him and I didnt notice, so Im just forwarding to group, in case anyone need.

---- Or you may try Union, one for green other for fairway ----