DECLARE @xml xml;
-- Create sample XML document
SET @xml = N'<authors> 
  <author id = "1">Fabio Claudio Ferracchiati</author>
  <author id = "2">Hugo Kornelis</author>
  <author id = "3">Rob Walters</author>
  <author id = "4">Lara Rubbelke</author>
  <author id = "5">Adam Machanic</author>
  <author id = "6">Michael Coles</author>
  <author id = "7">Robin Dewson</author>
  <author id = "8">Jan D. Narkiewicz</author>
  <author id = "9">Robert Rae</author>
</authors>
<books>
  <book title = "Pro T-SQL 2005 Programmer&apos;s Guide">
    <isbn>159059794X</isbn>
    <author>6</author>
  </book>
  <book title = "Accelerated SQL Server 2008">
    <isbn>1590599691</isbn>
    <author>3</author>
    <author>6</author>
    <author>7</author>
    <author>1</author>
    <author>8</author>
    <author>9</author>
  </book>
  <book title = "Pro SQL Server 2008 XML">
    <isbn>1590599837</isbn>
    <author>6</author>
  </book>
</books>';
-- Perform outer join
SELECT @xml.query('for $author in /authors/author 
  where fn:empty($author[@id = /books/book/author])
  return <author> { $author } </author>');