xquery for grouping data from xml file

Question

I am trying to run an xquery for my xml file. This is my input xml file:

<?xml version="1.0" ?>
<authors>
   <author><name> Steven</name>
     <title>advanced</title>
     <title>TCPIP</title>
   </author>
   <author><name> George</name>
     <title>DataMining</title>
     <title>Economic</title>
   </author>
   <author><name> Den</name>
    <title>TCPIP</title>
    <title>Economic</title>
   </author>
</authors>

I am trying to group each title and count the number of authors for each title. Books should be listed in increasing order of number of authors. For each book, output its title, the number of authors and the name of all its authors in alphabetic order. Finally, the output should be like this:

<bib>
 <book authors="1">
  <title>Advanced</title>
  <author>Steven</authhor>
 </book>
 <book authors="1">
  <title>DataMining</title>
  <author>George</author>
 </book>
 <book authors="2">
  <title>TCPIP</title>
  <author>Den</author>
  <author>Steven</author>
 </book>
 <book authors="2">
  <title>Economic</title>
  <author>Den</author>
  <author>George</author>
 </book>
</bib>

This is my code which doesn't work :(. is there any help. please if it is possible correct my code instead of writing a new code, because i want the answer be look like to my answer.

<bib>
{for $a in doc('test')//authors
let $T:={for $c in doc ('test')//authors/author[title=$a/author/title]
order by count($T)
return <book authors="{count($T)}">
{<title> {distinct-values($T/title)}
for $x in $T
order by $x/name()
return <author>{$x/name()}</author>}
</book>}
</bib>

I think the problem is for the part that I try to use the let command. As I know I can group data by let command. I tried for each title in XML file to grab all authors and after that count the number of authors and sort them by name. I use this link: http://basex.org/products/live-demo/ to test my answer.

Answer 1

As Daniel Haley suggested, you should do it the other way round, i.e. iterating over each title and looking for the authors which write this specific title. You can use distinct-values or group by. The following should work:

<bib>
{
  let $titles := distinct-values(doc('test')//title)
  for $t in $titles
  let $authors := doc('test')//author[title = $t]
  order by count($authors)
  return
    <book authors="{count($authors)}">
      <title>{$t}</title>
      {
        for $a in $authors
        order by $a/name
        return <author>{$a/name/string()}</author>
      }
    </book>
}
</bib>