RE: [xml-dev] Some random noise on rational type systems for XML

["Andrew Layman" <andrewl@microsoft.com>]

In XML there are three (not two) syntactic devices that people have at
various times asserted have some connection to the absence of a tuple in
the relational model.  

XML has:
	1.	Omission of an optional element.
	2.	Presence of an element having the attribute xsi:nil
	3.	Presence of an element having empty content.

SQL does make a distinction between omission and NULL in some contexts
(e.g. an UPDATE statement) but not in others (e.g. a rowset).

-----Original Message-----
From: Jeff Lowery [mailto:Jeff.Lowery@creo.com] 
Sent: Tuesday, May 06, 2003 3:45 PM
To: 'Amelia A.Lewis'; xml-dev@lists.xml.org
Subject: RE: [xml-dev] Some random noise on rational type systems for
XML

> The SQL people
> encountered this, and found a solution, which made them 
> heretics in the
> eyes of the relational true believers.  The problem is that 
> whenever you
> have a thing that has a value, it is often useful to be able to say
> "don't know" "not specified" "undefined" "null" or "nil".  W3C XML
> Schema introduces a mechanism for this.  So, umm, why?  XML 
> already has
> a way to say nothing.  Say nothing.  The empty string.  No data.  Not
> specified.  

I would say "data is undefined" and "data does not exist" are two
different
things.  SQL treats NULL different from an empty string.  For XML, null
would seem not to be necessary for numbers, since 'empty' would not
necessarily be interpreted as zero.

> Heh.  But this is already too long, and besides, I *enjoy*
> cliff-hangers, so let's just Tune In Next Week for Another 
> Bland Episode

Spring: the silly season.

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