List titles of books published by Addison-Wesley
<bib>
 {
   for $book in doc("bib.xml")/bib/book
   where $book/publisher = "Addison-Wesley"
   return
      $book/title 
  }
</bib>
This where clause could be replaced by an XPath predicate:
<bib>
 {
   for $book in doc("bib.xml")/bib/book[publisher="Addison-Wesley"]
   return
      $book/title 
 }
</bib>But where clauses can combine 
multiple variables from multiple documents
Adapted from XML Query Use Cases