XPath - Quering two XML documents

Question

I have have two xml docs:

XML1:

<Books>
     <Book id="11">
          .......
          <AuthorName/>
     </Book>
     ......
</Books>

XML2:

<Authors>
     <Author>
          <BookId>11</BookId>
          <AuthorName>Smith</AuthorName>
     </Author>
</Authors>

I'm trying to do the following:

Get the value of XML2/Author/AuthorName where XML1/Book/@id equals XML2/Author/BookId.

XML2/Author/AuthorName[../BookId = XML1/Book/@id]

Answer 1

The document function is your friend, here is a short tutorial how to combine multiple input files.

EDIT: Of course, that works only if your are using Xpath in an Xslt script.

Answer 2

An XPath 1.0 expression cannot refer to more than one XML document, unless the references to the additional documents have been set up in the context of the XPath engine by the hosting language. For example, if XSLT is the hosting language, then it makes its document() function available to the XPath engine it is hosting.

document($xml2Uri)/Authors/Author[BookId = $mainDoc/Books/Book/@id]

Do note, that even the main XML document needs to be referenced via another <xsl:variable>, named here $mainDoc.

The document() function is available only if Xpath is hosted by XSLT! This is not mentioned in the answer of Doc Brown and is misleading the readers.

An XPath 2.x expression may refer to any additional XML document using the XPath 2.0 doc() function.

for $doc  in /,
    $doc2 in doc(someUri)
  return
    $doc2/Authors/Author[BookId = $doc/Books/Book/@id]