Re: [ubl-lcsc] RE: [ubl-ndrsc] Functional Dependency and Normalizationpaper

["Eve L. Maler" <eve.maler@sun.com>]

Tim McGrath wrote:
> Burcham, Bill wrote:
>> In 1NF (in appendix A) at the point where you've found the one-to-many
>> relationship between "order" and "order item" and you've shown the 
>> need (in
>> the relational model) for the "linkage" from "order item" back to "order"
>> via the foreign key.  It seems worth noting at that point that this 
>> linkage
>> is explicit in the relational model but implicit in the (eventual) XML 
>> one.
>> This linkage between "order" and "order item" will be inferred from the
>> context -- an "order item" is associated with the "order" under which it
>> appears.  We'll see this again and again.
>>
> I see the terms linkage, relationship and association as fairly similar. 
> In a relational model these are implemented by foreign keys, in XML by 
> including the associated element within the container (sorry, the 
> language gets in the way here - maybe we should just get down to 
> diagrams!).  It is just different ways to implement the same concept. 
> Not sure i like 'explicit' and 'implicit' - we could just say that one 
> may be more elegant.

XML's primary means of association is containment, but it also allows 
for association by reference.  The built-in XML datatypes of ID and 
IDREF[S] are one way, and XSD provides another that it actually calls 
"keys", a la RDBMS's.  (Personally, though, I think it's still high-risk 
to use the XSD key mechanism.)  You can also invent any number of 
application-specific linking mechanisms, or even use the Web's URI 
paradigm for addressing resources.  The NDR group has been trying to get 
to the topic of how to do content-by-reference and it looks like we'll 
have to push this higher in the stack...

	Eve

-- 
Eve Maler                                        +1 781 442 3190
Sun Microsystems                            cell +1 781 883 5917
XML Web Services / Industry Initiatives      eve.maler @ sun.com