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From Florent Guillaume ...@nuxeo.com>
Subject Re: 3.1.3.1 Removing Items
Date Mon, 13 Aug 2007 16:36:49 GMT
Thomas Mueller wrote:
> 2) Should JCR support nullable properties?

I think a better question is, "does a system have a different notions 
for a null value and the absence of a property?".

> Some say yes, some no. Let's see what other systems do:
> * XML: No (an empty string is not null)

And there's no "null" attribute value.

> * Files: No (an empty file is not null)

And there's no "null" file.

> * HTTP: No
> * Properties files: No
> * Windows registry (if I may): No
> * LDAP: No? (not sure)

LDAP goes further, usually (but it depends on the attribute "syntax", ie 
its content model) empty strings are disallowed. And in any case there's 
no notion of null storage.

> * SQL: Yes (there is no other way to say 'the property is not set')

But in SQL you have a given schema for your table, and if in that schema 
you have a 'name; column then in a row its value is either a string, or 
NULL. You can't say "I don't want a 'name' column on that row". So for 
SQL in a given schema there's no difference between the absence of a 
property and a NULL.

> * Java Hash tables: Hashtable no, HashMap yes

In a HashMap there's no distinction between a null value a no value:
   HashMap.get(Object key)
    Returns the value to which the specified key is mapped in this 
identity hash map, or null if the map contains no mapping for this key.


So in *ALL* systems above, there is no distinction (apart from 
introspecting schemas -- but that's a different problem) between null 
values and absent properties. They're two facets of the same thing.

And as Roy explained, that's the whole point of "null". If someone wants 
additional semantics, then they have to use a different storage model.


Florent


-- 
Florent Guillaume, Director of R&D, Nuxeo
Open Source Enterprise Content Management (ECM)
http://www.nuxeo.com   http://www.nuxeo.org   +33 1 40 33 79 87

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