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Re: "=" operator vs. "IS"

From: <terry(at)ashtonwoodshomes(dot)com>
To: "'Michael A Nachbaur'" <mike(at)nachbaur(dot)com>,"'Stefan Weiss'" <spaceman(at)foo(dot)at>
Cc: <pgsql-sql(at)postgresql(dot)org>
Subject: Re: "=" operator vs. "IS"
Date: 2004-06-28 23:00:13
Message-ID: 016701c45d63$ac859e80$2766f30a@development.greatgulfhomes.com (view raw or flat)
Thread:
Lists: pgsql-sql
Think about a join where you do something like:
t1.f1 = t2.f1

If both columns were blank, would you want the join to succeed?  Probably not, if you did, then you
would potentially have a cartesian select.  This is not a good explanation of NULL non-equality, but
I thought it might be useful.

NOTE: A related topic is OUTER JOIN's which is how the above join would properly be implemented

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


> -----Original Message-----
> From: pgsql-sql-owner(at)postgresql(dot)org
> [mailto:pgsql-sql-owner(at)postgresql(dot)org]On Behalf Of Michael A Nachbaur
> Sent: Monday, June 28, 2004 6:28 PM
> To: Stefan Weiss
> Cc: pgsql-sql(at)postgresql(dot)org
> Subject: Re: [SQL] "=" operator vs. "IS"
>
>
> On June 28, 2004 03:02 pm, Stefan Weiss wrote:
> > I'm just curious - why is it not possible to use the "=" operator to
> > compare values with NULL? I suspect that the SQL standard specified
> > it that way, but I can't see any ambiguity in an expression
> like "AND
> > foo.bar = NULL". Is it because NULL does not "equal" any
> value, and the
> > expression should be read as "foo.bar is unknown"? Or is
> there something
> > else I'm missing?
>
> As far as I have been able to tell, it is one of those quirks
> about SQL that
> you shouldn't bother trying to understand.  It just IS.  <rimshot/>
>
> --
> Michael A. Nachbaur <mike(at)nachbaur(dot)com>
> http://nachbaur.com/pgpkey.asc
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster
>


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Subject: Re: FW: "=" operator vs. "IS"
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