Re: Query on the XML graph

Thank you very much. I am now much more clearer now.

Regards,
Fianny

On Sun, 8 Feb 2004, Michael Brundage wrote:

>
> XPath can follow ID/IDREF edges (in one direction) using the id() function.
>
> Also, XPath can do self-joins, and when extended with the document() (or
> doc()) function, it can do cross-document joins.  The kinds of joins that
> can be expressed are limited by XPath's inability to introduce variables
> (aliases).
>
> Some joins are built-in to the semantics of XPath.  For example,
> //A[B/C > B/D]
> Iterates over the B children of A twice (independently, as a cross-product).
>
>
> Other joins are possible using absolute paths in predicates to achieve
> independence.
>
> For example, given a structure containing Employee elements with ID and
> ReportsTo attributes (typed as ID, IDREF, respectively), here are some joins
> expressed using XPath 1.0:
>
> (: the manager of a particular employee :)
> Id(//Employee[@ID='X1']/@ReportsTo)
>
> (: all managers - i.e., all employees with at least one direct report :)
> //Employee[//Employee/@ReportsTo = @ID]
>
> (: employees who report to people not in the organization, or report to no
> one :)
> //Employee[not(@ReportsTo = //Employee/@ID)]
>
> (: their direct reports :)
> //Employee[@ReportsTo = //Employee[not(@ReportsTo = //Employee/@ID)]/@ID]
>
> etc.
>
> Other joins are difficult to express, require unnatural syntax, or are
> outright inexpressible using XPath 1.0:
>
> (: all employees with at least two direct reports
>   You'd like to do: :)
> //Employee[count(//Employee[@ReportsTo = $outer/@ID]) >= 2]
> (: or :)
> //Employee[count(id(//Employee/@ReportsTo)[@ID = $outer/@ID]) >= 2]
> (: but you have no way to introduce an alias to the outer Employee.
> If XPath had an idref() function, you could do something like: :)
> //Employee[count(idref(@ID)) >= 2]
>
>
> (: managers whose entire organization is a certain size
>  - there's no way to count subtrees unless the hierarchy already represents
> the relationship.  For example, :)
> //Employee[count(.//Employee)]
> (: is trivial, but there's no way to recursively follow ID/IDREF edges :)
>
> etc.
>
>
>
> Hope this helps,
>
> Michael Brundage
> [email protected]
>
> Writing as
> Author, XQuery: The XML Query Language (Addison-Wesley, 2004)
> Co-author, Professional XML Databases (Wrox Press, 2000)
>
> not as
> Technical Lead
> Common Query Runtime/XML Query Processing
> WebData XML Team
> Microsoft
>
>
>
> On 2/8/04 7:16 PM, "Jim Tivy" <[email protected]> wrote:
> > One way to accomplish cross document links is use foreignkey/primary key
> > relationships and then do joins using XQuery.
> >
> >> -----Original Message-----
> >> From: [email protected] [mailto:[email protected]]On Behalf Of
> >> Jiang Ming Fei
> >> Sent: Sunday, February 08, 2004 6:48 PM
> >> To: [email protected]
> >> Cc: [email protected]
> >> Subject: Query on the XML graph
> >>
> >>
> >>
> >> I am now doing the research on the XML graph. What I am wondering about is
> >> that how the nowaday query engines cope with the XML files that have idref
> >> attributes, or XML graph? Since XPath have no notation for the idref/id
> >> edge. Or I miss some details?  So can I always treat the XML document as
> >> a tree with the special node, i.e. idref node?
> >>
> >> Thank you in advance for your suggestion.
> >>
> >> Regards,
> >> Fianny
> >>
> >>
> >
>
>