let $ms:=doc("movies_alone.xml"),
    $as:=doc("artists_alone.xml")

return <results> {

for $m in $ms//movie
let $d := $as//artist[@id=$m/director/@id]/(first_name|last_name)
return <movie>
  {$m/title,
  <director>{$d}</director>
  }
</movie>
} </results>