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From: Ross J. Reedstrom <reedstrm@wallace.ece.rice.edu>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: pgsql-sql@postgresql.org
Subject: Re: [SQL] NULL
Date: Tue, 30 Nov 1999 13:09:06 -0600
Message-ID: <19991130130906.D6156@wallace.ece.rice.edu> (raw)
In-Reply-To: <21582.943985242@sss.pgh.pa.us>
References: <19991130101246.B6156@wallace.ece.rice.edu>
	<21582.943985242@sss.pgh.pa.us>

On Tue, Nov 30, 1999 at 01:07:22PM -0500, Tom Lane wrote:

<snipped excellent description about the problem with NULL as a
column constraint>

Ah, now I understand. And as I said, if it would cause a problem, now or
in the forseeable future, skip it. I propose dropping Tom's explanation
(or a summary of it) into the FAQ. how about :

------------
Q: Why doesn't PostgreSQL support NULL as a column constraint?

Short A: Because it's not in the SQL92 standard.

Long A:

Because of SQL's rather silly decision not to put any punctuation between
column constraint clauses, anything that can start a column constraint
clause can't also be a valid entry in an expression.  Consider

      CREATE TABLE foo (field1 int DEFAULT 2 ++ NULL)

Here, the grammar cannot figure out whether the default expression is
"2 ++ NULL" (with ++ an infix operator) or "2 ++" (++ a postfix operator)
and NULL a separate constraint clause.
------------

Ross
-- 
Ross J. Reedstrom, Ph.D., <reedstrm@rice.edu> 
NSBRI Research Scientist/Programmer
Computer and Information Technology Institute
Rice University, 6100 S. Main St.,  Houston, TX 77005

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