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RE: XML vs relational database

• From: noah_mendelsohn@u...
• To: "Michael Kay" <mike@s...>
• Date: Thu, 16 Aug 2007 09:59:49 -0400

Also, I think it's useful to consider separately collections of "well 
formed" XML, vs. collections known to be valid per some schema. 
Collections of well formed documents behave as you describe:  because they 
vary a great deal in structure, you generally have to interrogate the 
structure of each instance if you care, and to a significant degree the 
structures are self describing.  I think that a SQL table more closely 
resembles a collection of documents known to be valid per some schema. 
Depending on how rigid the constraints imposed by the schema are, it may 
or may not tightly bound the structure of the instances.  Still, it's 
reasonable to ask:  can a query interrogate the schema, which is analagous 
I think to querying the structure of SQL tables.  The answer in XML will 
depend on which schema technologies and query systems you adapt.  I will 
say that one reason that XML Schema goes to such trouble to formalize not 
just its transfer syntax but also it's so-called "component model" is to 
make possible exposing the semantics of the schema to query systems. 
What's missing, I should say, is a standard XML serialization that's quite 
ismorphic to those components.  What we have today for a transfer syntax 
is more analagous to the SQL statements that would define a table, rather 
than schema table that would describe its columns.  Schema components are 
analagous to the latter, and are quite carefully set out.  There is no 
standard transfer syntax isomorphic to those components at this time, 
although nonstandard versions have been put to good use (e.g. the -r 
switch on XSV).

Noah

--------------------------------------
Noah Mendelsohn 
IBM Corporation
One Rogers Street
Cambridge, MA 02142
1-617-693-4036
--------------------------------------








"Michael Kay" <mike@s...>
08/16/2007 09:17 AM
 
        To:     "'Sylvain LOISEAU'" <sylvain.loiseau@w...>, 
<xml-dev@l...>
        cc:     (bcc: Noah Mendelsohn/Cambridge/IBM)
        Subject:        RE:  XML vs relational database


> Don't you think there is a structural 
> difference, from this point of view, between XML and DBMS?
> 

Sure, in SQL all the instances have the same structure, so the structure
information can be stored separately from the instances.

Michael Kay
http://www.saxonica.com/


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• References:
  • RE: XML vs relational database
    • From: "Michael Kay" <mike@s...>