[xquery-talk] [xml-dev] Re: JDBC in an XQuery world (?)

[Ronald Bourret]

I agree with Peter here: SQL/XML is already a fairly complete solution.

For those of you not familiar with SQL/XML, it does a couple of things:

1) It defines a set of functions that allow you to return relational 
data as XML. This is the relational => XML mapping.

2) It defines a primitive XML data type. That is, you can have a column 
whose type is XML and store XML documents in that column.

3) XMLTABLE allows you to use XPath-like constructs to define what parts 
of an XML document (either stored in an XML column or constructed on the 
fly) correspond to a table. You then use SQL to query that "table." (See 
Peter's example below.) This is the XML => relational mapping.

In the case of XML databases, a collection in the XML database would 
correspond to an XML column in a relational database. This could be 
queried via XMLTABLE (which maps the XML documents to relational tables) 
and SQL (which queries those tables). (I can't remember whether 
XMLTABLEs are updateable -- I suspect not -- so this doesn't address the 
update side of the question.)

As a side note, it's misleading to think of "XQuery databases" here. 
XQuery is, at most, an implementation detail. That is, the class of 
drivers you are talking about would work with different XML data 
sources: XML databases, XML documents in the file system, URLs that 
return XML, etc.

How you implement such drivers depends on the data source and the amount 
of work you want to do. For example, to implement such a driver over an 
XML database that supports XQuery, you could map SQL to XQuery or you 
could just use XPath to extract data from the database, construct tables 
from that data, and use a lightweight SQL engine to further query that 
data. (Not efficient, but easier to implement, and similar to what JDBC 
drivers over CSV do today.)

-- Ron

P.S. For a list of products that already do this sort of thing, see:

    http://www.rpbourret.com/xml/ProdsWrappers.htm

The URLs are all out of date, but the descriptions give a flavor of what 
has already been done.

Peter Coppens wrote:
> On 02 Oct 2011, at 20:42, John Cowan wrote:
> 
>> Liam R E Quin scripsit:
>>
>>>> What if I have (or want) an XQuery database
>>>> (or would you call that a XML database with an XQuery interface ...?) 
>>> SQL/XML I think describes enough of a mapping, no?
>> I don't think so.  SQL/XML extends SQL to add XML as a new kind of primitive
>> SQL data type, like strings, numbers, and dates.  What's needed here is a
>> way to do ordinary SQL operations over an XDM-based database.
> 
> It does do a bit more as far as I understood it, e.g. it also introduce a "schredding" construct, XMLTABLE
> 
> Here is an example (http://www.ibm.com/developerworks/data/library/techarticle/dm-0708nicola/)
> 
> SELECT X.* 
> FROM emp, 
> XMLTABLE ('$d/dept/employee' passing doc as "d" 
>    COLUMNS 
>    empID 	INTEGER 	PATH '@id',
>    firstname 	VARCHAR(20) 	PATH 'name/first',
>    lastname 	VARCHAR(25) 	PATH 'name/last') AS X
> 
> In this case the source is still a table with some XML in some column but I guess making the step to some XML collection in some XML database is not that big.
> 
> Peter
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