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

From: Todd Vierling <tv(at)pobox(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: pgsql-bugs(at)postgreSQL(dot)org
Subject: Re: [BUGS] (null) != (null) ?
Date: 1999-10-26 14:08:01
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-bugs
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);

(0 rows)

Now *that* is awfully disturbing.  :>

-- Todd Vierling (tv(at)pobox(dot)com)

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Next:From: Todd VierlingDate: 1999-10-26 14:41:52
Subject: Re: [BUGS] (null) != (null) ?
Previous:From: Панков СвятославDate: 1999-10-26 13:33:04
Subject: it's possible bug

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