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From: "Jose' Soares" <jose(at)sferacarta(dot)com>
To: Bruce Momjian <maillist(at)candle(dot)pha(dot)pa(dot)us>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, lockhart(at)alumni(dot)caltech(dot)edu, hackers(at)postgresql(dot)org
Subject: Re: [HACKERS] Re: NULL & NOT NULL
Date: 1998-12-29 14:43:13
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Bruce Momjian wrote:
> > "Thomas G. Lockhart" <lockhart(at)alumni(dot)caltech(dot)edu> writes:
> > >>>>>> create table authors (
> > >>>>>> zip             char(5)         null
> > >>>>>> );
> >
> > > Sheesh. After that long song and dance about why we can't implement
> > > this, it turns out that it works fine. We had been trying to implement a
> > > slightly different syntax, "WITH NULL", which conflicted with the
> > > SQL92-defined data type declaration "TIMESTAMP WITH TIME ZONE".
> >
> > > The "Practical SQL Handbook"-compatible form will be available in the
> > > next full release of Postgres. Thanks.
> >
> > Now that we have the syntax problem straightened out: I'm still confused
> > about the semantics.  Does a "NULL" constraint say that the field
> > *must* be null, or only that it *can* be null (in which case NULL is
> > just a noise word, since that's the default condition)?  I had assumed
> > the former, but Bruce seemed to think the latter...
> Can be null.  Noise word.  At least that is what I rememeber Thomas
> saying, and because it was noise, we removed it.  In fact, it doesn't
> look like the standard accepts it, but there is no reason we can't.

This NULL clause is not part of constraints it is a default option and
we already support it,
there's nothing like: 
CREATE TABLE table1 (field1 type NULL) in SQL92.

but the following is SQL92 and it works on PostgreSQL:
prova=> CREATE TABLE table1 (field1 INTEGER DEFAULT NULL);

SQL92 syntax:

         11.5  <default clause>


         Specify the default for a column or domain.


         <default clause> ::=
                DEFAULT <default option>

         <default option> ::=
              | <datetime value function>
              | USER
              | CURRENT_USER
              | SESSION_USER
              | SYSTEM_USER
              | NULL


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