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