Query with let

• let assigns a variable for reuse.

For each book in the bibliography, list the difference between the book's price and the average price.

<results> 
  {
   let $doc := doc("bib.xml")
   let $average := avg($doc//price)
   for $b in $doc/bib/book
     let $difference := $b/price - $average
     return
       <data>{ $b/title } is {$difference} more expensive than the average. </data>
  }    
</results>