W3C XQuery 3.0 - Group By Use Cases Q2, using map:map

gistfile1.xq

<result>{
  let $m := map:map()
  let $build :=
    for $sales in doc("sales-records.xml")/*/record
    let $state := doc("stores.xml")/*/store[
      store-number = $sales/store-number]/state
    let $category := doc("products.xml")/*/product[
      name = $sales/product-name]/category
    let $key := concat($state, '|', $category)
    return map:put(
      $m, $key, sum((
        map:get($m, $key),
        $sales/qty)))
  for $key in map:keys($m)
  let $state := element state {
    substring-before($key, '|') }
  let $category := element category {
    substring-after($key, '|') }
  order by $state, $category
  return
    <group>
      {$state, $category}
      <total-qty>{map:get($m, $key)}</total-qty>
    </group>
}</result>