Mailbox-Line: From sszabo@megazone23.bigpanda.com  Sun Jul  7 16:45:03 2002
Date: Sun, 7 Jul 2002 13:44:54 -0700 (PDT)
From: Stephan Szabo <sszabo@megazone23.bigpanda.com>
To: Roy Souther <roy@silicontao.com>
Cc: <pgsql-sql@postgresql.org>
Subject: Re: Bad SUM result
In-Reply-To: <200207071425.56235.roy@silicontao.com>
Message-ID: <20020707133824.L19117-100000@megazone23.bigpanda.com>
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII

On Sun, 7 Jul 2002, Roy Souther wrote:

> I have an invoice database that has two tables one for invoices and one for
> payments. I want to get the account balance for a client by subtracting the
> sum of all payments from the sum off all invoices for that client.
>
> Here is the SQL that I thought should work.
> SELECT SUM(t0.totalprice)-SUM(t1.paymentamount) FROM invoices t0, payments t1
> WHERE t0.custnumber='1' AND t1.custnumber='1'
>
> It works fine if there is only one invoice and one payment but as soon as
> there is more then one of either it screws up. For each match found in
> payments the invoice sum is added to the total. So if client 1 purchased a
> $100 item then maid a $10 payment the SQL would return the balance of $90
> just fine. When the client makes a second payment of $15 the balance is $75
> but this SQL returns ($100+$100)-($10+$15) = $175. A third payment of $1
> would return ($100+$100+$100)-($10+$15+$1) = $274.

Right, because you're doing a join that ends up with
100 | 10
100 | 15
100 | 1

as the effective rows.


> Could some one explain this to me and recommend an SQL command that would work
> please? I could do this using a temp table but that would be very messy as I
> would really like it to be a single SQL command.

Maybe something like:
select sum(amt) from (select custnumber, totalprice as amt from
invoices union select custnumber, -paymentamount as amt from payments) as
c where custnumber='1';