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Cc: "pgsql-sql@postgresql.org" <pgsql-sql@postgresql.org>
From: David Johnston <polobo@yahoo.com>
Subject: Re: conditional FROM
Date: Sat, 10 Dec 2011 11:28:29 -0500
To: Richard Klingler <richard@klingler.net>

On Dec 10, 2011, at 11:03, Richard Klingler <richard@klingler.net> wrote:

> Good day...
> 
> I'm trying to build a query for PGSQL 9.1 where a table has two 
> references with only one being used depending of the type of entry..
> 
> For example, the table has following simplified structure:
> 
>    portid        primary key
>    port2node    index to table node
>    port2card    index to table card
> 
> So how can I do a conditional FROM clause in the query depending on the 
> column port2node and port2card?
> If port2card is Null or 0 I don't want it in the FROM clause as the 
> query will return unneccessary duplicate
> row...the same goes for port2node being Null or 0...
> 
> 
> thanx in advance
> richard
> 
> 

Two options (one of which may not work for you).

1. Write two queries, one for each table, and union the results.
2. Use LEFT JOINs (somehow...)

David J.