W3C XQuery 3.0 - Group By Use Cases Q1, using map:map

gistfile1.xq

<sales-qty-by-product>{
  let $m := map:map()
  let $build :=
    for $sales in doc("sales-records.xml")/*/record
    let $pname := $sales/product-name
    return map:put(
      $m, $pname, sum((
        map:get($m, $pname),
        $sales/qty)))
  for $pname in map:keys($m)
  order by $pname
  return
    <product name="{$pname}">{
      map:get($m, $pname) }</product>
}</sales-qty-by-product>