From: terry@greatgulfhomes.com
To: "'Stephan Szabo'" <sszabo@megazone.bigpanda.com>,
	"'Don Drake'" <dondrake@gmail.com>
Cc: "'Richard Huxton'" <dev@archonet.com>, <pgsql-sql@postgresql.org>
Subject: Re: NULLS and string concatenation
Date: Fri, 19 Nov 2004 19:16:53 -0500
Message-ID: <00c201c4ce96$4c56f500$2766f30a@development.greatgulfhomes.com>
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In-Reply-To: <20041119105650.E40388@megazone.bigpanda.com>

There is an easy solution anyway, use coalesce to ensure you are never returning a null result for
any components of the concat.

e.g.
select 'some text, blah:' || coalesce(NULL, '')
equates to 'some text, blah:' || ''
hence
'some text, blah:'

Terry Fielder
Manager Software Development and Deployment
Great Gulf Homes / Ashton Woods Homes
terry@greatgulfhomes.com
Fax: (416) 441-9085


> -----Original Message-----
> From: pgsql-sql-owner@postgresql.org
> [mailto:pgsql-sql-owner@postgresql.org]On Behalf Of Stephan Szabo
> Sent: Friday, November 19, 2004 2:04 PM
> To: Don Drake
> Cc: Richard Huxton; pgsql-sql@postgresql.org
> Subject: Re: [SQL] NULLS and string concatenation
>
>
>
> On Fri, 19 Nov 2004, Don Drake wrote:
>
> > On Fri, 19 Nov 2004 17:48:34 +0000, Richard Huxton
> <dev@archonet.com> wrote:
> > > Don Drake wrote:
> > > > select 'some text, should be null:'|| NULL
> > > >
> > > > This returns NULL and no other text.  Why is that?  I
> wasn't expecting
> > > > the "some text.." to disappear altogether.
> > > >
> > > > Is this a bug?
> > >
> > > No. Null is "unknown" if you append unknown (null) to a
> piece of text,
> > > the result is unknown (null) too.
> > >
> > > If you're using NULL to mean something other than
> unknown, you probably
> > > want to re-examine your reasons why.
> > >
> >
> > I'm using NULL to mean no value.  Logically, NULL is
> unknown, I agree.
> >
> > I'm trying to dynamically create an INSERT statement in a function
> > that sometimes receives NULL values.
> >
> > This is still strange to me.  In Oracle, the same query would not
> > replace the *entire* string with a NULL, it treats the NULL as a no
> > value.
>
> Oracle has some incompatibilities with the SQL spec (at least
> 92/99) wrt
> NULLs and empty strings so it isn't a good comparison point.
> The spec is
> pretty clear that if either argument to concatenation is NULL
> the output
> is NULL.
>
> > I can't find in the documentation where string concatenation of any
> > string and NULL is NULL.
>
> I'm not sure it does actually.  I'd have expected to see some
> general text
> on how most operators return NULL for NULL input but a quick
> scan didn't
> find any.
>
>
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