Re: Document NULL

On Fri, May 3, 2024 at 1:14 AM jian he <jian(dot)universality(at)gmail(dot)com> wrote:

> On Fri, May 3, 2024 at 2:47 PM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
> wrote:
> >
> > On Thu, 2024-05-02 at 08:23 -0700, David G. Johnston wrote:
> > > Version 2 attached. Still a draft, focused on topic picking and
> overall structure.
> >
> > I'm fine with most of the material (ignoring ellipses and typos), except
> this:
> >
> > + The NOT NULL column constraint is largely syntax sugar for the
> corresponding
> > + column IS NOT NULL check constraint, though there are metadata
> differences
> > + described in create table.
> >
>
> the system does not translate (check constraint column IS NOT NULL)
> to NOT NULL constraint,
> at least in domain.
>
>
I'll change this but I was focusing on the fact you get identical
user-visible behavior with not null and a check(col is not null). Chain of
thought being we discuss the is not null operator (indirectly) already and
so not null, which is syntax as opposed to an operation/expression, can
leverage that explanation as opposed to getting its own special case. I'll
consider this some more and maybe mention the catalog dynamics a bit as
well, or at least point to them.

> drop domain connotnull cascade;
> create domain connotnull integer;
> alter domain connotnull add check (value is not null);
> \dD
>

This reminds me, I forgot to add commentary regarding defining a not null
constraint on a domain but the domain type surviving a left join but having
a null value.

David J.