Date: Sun, 24 Aug 2003 03:28:15 +0200
From: Bertrand Petit <pgsql-sql@phoe.frmug.org>
To: PostgreSQL-sql <pgsql-sql@postgresql.org>
Subject: Equality operators on NULL values
Message-ID: <20030824032815.A32448@memo.frmug.org>
Mail-Followup-To: PostgreSQL-sql <pgsql-sql@postgresql.org>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
User-Agent: Mutt/1.2.5.1i


	(in)Equality operators evaluates to NULL when any of their
arguments are NULL as summarized bellow.

	NULL=NULL ==> NULL
	1=NULL    ==> NULL
	1=1       ==> t
	1=2       ==> f

I have a case where I need to delete rows from table A for records
that are designated in table B. One of the joined colum can have NULL
values. Therefore I need an operator behaving as follows:

	NULL=NULL ==> t
	1=NULL    ==> NULL (or anything else as long as it is not t)
	1=1       ==> t
	1=2       ==> f

I replaced the use of the = operator with this expression:

	nullif(tabA.col_with_nulls, tabB.col_with_nulls) IS NULL

It works correctly but the intent is not that clear.

	Is there a stock replacement operator that would behave like
the second truth table?

	Regards,
	Bertrand.

-- 
%!PS
297.6 420.9 translate 90 rotate 0 setgray gsave 0 1 1{pop 0 180 moveto 100
180 170 100 170 -10 curveto 180 -9 180 -9 190 -10 curveto 190 100 100 180
0 180 curveto fill 180 rotate}for grestore/Bookman-LightItalic findfont
240 scalefont setfont -151.536392 -63.7998886 moveto (bp)show showpage