xml-dev - Re: parser for xml-data?

[rbourret@dvs1.informatik.tu-darmstadt.de (Ron Bourret)]

Sudarshan Purohit wrote:

>     I think this is a good place to talk of the project I'm doing right now.
> Basically, it's a translator between an XML-Data document and any
> ODBC interface. This involves :
> 1. Taking an input XML-Data file and outputting a series of SQL
> statements that result in the same information being stored in an RDBMS.

I'm working on a similar project (moving data from XML to relational databases 
and vice versa).

Do you really mean you are moving data from an XML-Data document?  XML-Data is 
an XML language for describing metadata, including DTDs.  As such, the data in 
an XML-Data file is metadata, not data.  This is suitable if you want to create 
tables in the RDBMS that match the XML-Data schema.  However, an XML-Data file 
won't give you any data to put in those tables.

By the way, there was a question the other day about how one might actually 
deploy XML-Data as a substitute for a DTD.  Was this ever answered?

> 2. Using an ODBC Recordset interface to get an equivalent XML-Data
> file. ( I'm still working out this one)
>     The first part involves parsing the XML file, so I might be able to help 
out
> somewhere, Yosr.
> 
> While we're on the topic, I'd like to ask a few things.
> 1. XML-Data stores Data as "entities". This implies that we've got
> a more abstract representaion of data than what would have been
> possible through relational databases. For example, we've got this
> "ONEORMORE,ZEROORMORE" occurence attribute for child elements.
> Implementing this in an RDBMS would mean creating another linked
> table for that data item. The same goes for complex data types.
> Am I on the right track?

You are correct.  XML describes an object model, so the problem is roughly the 
same as moving object data to and from an RDBMS -- you might want to look at 
some papers on the subject.  There is an excellent white paper introducing the 
subject at:

   http://www.ontos.com/mapcon.htm
   
Another good paper is:

   Shekar Ramanathan, Julia E. Hodges: Extraction of Object-Oriented Structures 
from Existing Relational Databases, SIGMOD Record, Vol. 26, Number 1, March 
1997, pp. 59-64.
   
   Postscript: http://bunny.cs.uiuc.edu/sigmod/sigmod_record/9703/rama.ps

The only major difference I have found so far for XML is that the elements in 
XML documents are ordered, while data members in OO programming languages are 
not.

> 
> 2. If we're creating XML specifically for this purpose, it'll have to
> be in the format
> 
> <elementType id="Name">
> ....</elementType>
> 
> <elementType id="SerialNo">
> ....</elementType>
> 
> <elementType id="Tuple">
> <!--contains Name and Serial No: as children, occuring once.
> Also all key declarations, constarints, .... of teh table as such-->
> .....
> </elementType>
> 
> <elementType id="TheRealTable">
> <element type="#Tuple" occurs="ONEORMORE">
> </elementType>
> 
> This restrictive structure, of course, means that you end up
> not using this kind of document for anything else. Am I right
> in thinking that this has to be done?

Again, this makes sense if you are storing schema information, but I'm not sure 
that that is what you want.  The problem is one of creating a mapping from the 
DTD (expressed as a DTD, XML-Data, or another competing schema language) to 
tables/columns and then using that mapping to move data from an XML file (which 
uses that DTD) to the RDBMS.  You can certainly do this for a large number of 
DTDs -- I'm not yet convinced it is possible for all.

> 
> 3. The XML-Data specification says that it's open-ended
> and new types of entities may be added. I'm assuming this means
> I'm able to add  lines like
> 
> <level lev="COLUMN">
> or <level lev="TUPLE">
> into my element delcarations, which will be ignored by other
> parsers than mine. Am I right ?
> 
> I came rather late into the discussion, so it's possible that
> this stuff may have been figured out already. If so, tell me.
> 
> Thanks,
> 
> Sudarshan                             1030hrs.

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