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Re: [BUGS] (null) != (null) ?

From: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
To: Todd Vierling <tv(at)pobox(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-bugs(at)postgreSQL(dot)org
Subject: Re: [BUGS] (null) != (null) ?
Date: 1999-10-26 17:44:02
Message-ID: 199910261744.NAA28154@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugs
I have just added a paragraph about this comparison in my book.  You can
see it on the documentation web page under "Published book".


> On Tue, 26 Oct 1999, Tom Lane wrote:
> 
> : > Both "fieldname" definitions are identical (verified with char(2) and
> : > varchar(100) in particular), and both tables contain a row with a "null" in
> : > that field.  However, the results don't contain the row with the "null"
> : > value.
> : 
> : NULL = NULL does not yield TRUE, it yields NULL.  For that matter,
> : NULL != NULL does not yield FALSE --- it yields NULL.  This is a
> : basic consequence of the semantics of NULL.
> 
> !?
> 
> I have been using such constructs on commercial databases for ages.  Do you
> have a link to a web-based SQL standard transcription that I could look this
> up?  (I'll check up on exactly which database(s) I can use this type of
> construct when I get back to work tomorrow....)
> 
> It seems _extremely_ counter-intuitive, especially in cases where both
> fields are in fact the same type.
> 
> : Nearly all Postgres operators yield NULL if any input is NULL.
> 
> Interesting ... so see my clarification of (2) below.
> 
> : If you really want to match up nulls in your example, you can do
> : something like
> : 	WHERE (a.fieldname = b.fieldname) OR
> : 	      (a.fieldname IS NULL AND b.fieldname IS NULL)
> 
> Which I already described in my text, sigh.
> 
> : This is pretty grotty, of course, so my inclination would be to
> : use a special non-NULL value --- an empty string, for example ---
> 
> Doesn't work for datetime, which is an important application in my case
> which rather needs null to indicate "no datestamp at all".
> 
> : > (2) NOT IN doesn't seem to work at all.  I always get 0 results--and very
> : >     rapidly at that!--regardless of the situation.
> : 
> : I don't think it's quite *that* broken.  How about a concrete
> : example of what you're trying to do?
> 
> Well, after reading your statement about "Nearly all Postgres ...", here's a
> very simple example that I was able to create based on that assumption:
> 
> => create temp table foo (name varchar(10));
> => create temp table foo2 (name varchar(10));
> => insert into foo values (null); // <<- here's the tripwire!
> => insert into foo values ('a');
> => insert into foo2 values ('a');
> => insert into foo2 values ('b');
> => select * from foo2 where field not in (select field from foo);
> 
> field
> -----
> (0 rows)
> 
> Now *that* is awfully disturbing.  :>
> 
> -- 
> -- Todd Vierling (tv(at)pobox(dot)com)
> 
> 
> ************
> 
> 


-- 
  Bruce Momjian                        |  http://www.op.net/~candle
  maillist(at)candle(dot)pha(dot)pa(dot)us            |  (610) 853-3000
  +  If your life is a hard drive,     |  830 Blythe Avenue
  +  Christ can be your backup.        |  Drexel Hill, Pennsylvania 19026

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