Querying XML: Use Cases - Use case PARTS: recursive parts explosion.
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This use case illustrates how a recursive query might can construct a hierarchical document of arbitrary depth from flat structures stored in a database.

This use case is based on a “parts explosion” database that contains information about how parts are used in other parts.

The input to the use case is a “flat” document in which each different part is represented by a<part>element withpartidand name attributes. Each part may or may not be part of a larger part; if so, thepartidof the larger part is contained in apartof attribute. This input document might be derived from a relational
database in which each part is represented by a table row withpartidas primary key andpartofas a foreign key referencingpartid.

The challenge of this use case is to write a query that converts the “flat” representation of the parts explosion, based on foreign keys, into a hierarchical representation in which part containment is represented by the document structure.

The input data set uses the following DTD:

  <!DOCTYPE partlist[
      <!ELEMENT partlist (part*)>
      <!ELEMENT part EMPTY>
      <!ATTLIST part
           partid   CDATA   #REQUIRED
           partof   CDATA   #IMPLIED
           name     CDATA   #REQUIRED>
  ]>

Although thepartidandpartofattributes could have been of type ID and IDREF, respectively, in this schema they are treated as character data, possibly materialized in a straightforward way from a relational database. Eachpartofattribute matches exactly onepartid. Parts having nopartofattribute are not contained in any other part.

The output data conforms to the following DTD:

  <!DOCTYPE parttree [
      <!ELEMENT parttree (part*)>
      <!ELEMENT part (part*)>
      <!ATTLIST part
            partid  CDATA  #REQUIRED
            name    CDATA  #REQUIRED>
  ]>

Sample data conforming to that DTD might look like this:

  <?xml version="1.0" encoding="ISO-8859-1"?>
  <partlist>
    <part partid="0" name="car"/>
    <part partid="1" partof="0" name="engine"/>
    <part partid="2" partof="0" name="door"/>
    <part partid="3" partof="1" name="piston"/>
    <part partid="4" partof="2" name="window"/>
    <part partid="5" partof="2" name="lock"/>
    <part partid="10" name="skateboard"/>
    <part partid="11" partof="10" name="board"/>
    <part partid="12" partof="10" name="wheel"/>
    <part partid="20" name="canoe"/>
  </partlist>
Question 1. Convert the sample document from "partlist" to "parttree" format (see
the DTD section for definitions). In the result document, part containment is
represented by containment of one <part> element inside another. Each part that
is not part of any other part should appear as a separate top-level element in
the output document:
<xsl:template match="partlist">
  <parttree>
    <!-- Start with the part that is not part of anything -->
   <xsl:apply-templates select="part[not(@partof)]"/>
  </parttree>
</xsl:template>

<xsl:template match="part">
  <part partid="{@partid}" name="{@name}">
    <xsl:apply-templates select="../part[@partof = current()/@partid]"/>
  </part>
</xsl:template>
It turns out that this sort of transformation is easier to code and understand in
XSLT than in XQuery. For comparison, here is the XQuery solution offered by the
W3C paper:
define function one_level (element $p) returns element
{
   <part partid="{ $p/@partid }"
         name="{ $p/@name }" >
       {
          for $s in document("partlist.xml")//part
          where $s/@partof = $p/@partid
          return one_level($s)
       }
   </part>
}

<parttree>
  {
    for $p in document("partlist.xml")//part[empty(@partof)]
    return one_level($p)
  }
</parttree>

Even without a detailed understanding of XQuery, you should be able to see that the XQuery solution is needed to explicitly implement the recursion while XSLT’sapply-templatesand pattern matching allow a more declarative solution. Granted, the difference is not that dramatic, but I find XSLT more elegant for this type of problem.

Next: Use case REF: queries based on references. >>
 

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