How to retrieve largest value from duplicate nodes in XQuery?

Question

I have an XML file. I wanted to retrieve the largest value node from the duplicate values (in the case below, author, title, and genre fields are the same, but the number field is different).

<?xml version="1.0"?>
<catalog>
   <book>
      <author>Gambardella, Matthew</author>
      <title>XML Developer's Guide</title>
      <genre>Computer</genre>
      <number>1234</number>
   </book>
   <book>
      <author>Ralls, Kim</author>
      <title>Midnight Rain</title>
      <genre>Fantasy</genre>
      <number>1235</number>
   </book>
   <book>
      <author>Corets, Eva</author>
      <title>Maeve Ascendant</title>
      <genre>Fantasy</genre>
      <number>1236</number>
   </book>
   <book>
      <author>Corets, Eva</author>
      <title>Oberon's Legacy</title>
      <genre>Fantasy</genre>
      <number>1237</number>
   </book>
   <book>
      <author>Corets, Eva</author>
      <title>The Sundered Grail</title>
      <genre>Fantasy</genre>
      <number>1238</number>
   </book>
   <book>
      <author>Randall, Cynthia</author>
      <title>Lover Birds</title>
      <genre>Romance</genre>
      <number>1239</number>
   </book>
   <book>
      <author>Thurman, Paula</author>
      <title>Splish Splash</title>
      <genre>Romance</genre>
      <number>1240</number>
   </book>
   <book>
      <author>Thurman, Paula</author>
      <title>Splish Splash</title>
      <genre>Romance</genre>
      <number>1246</number>
   </book>
   <book>
      <author>Thurman, Paula</author>
      <title>Splish Splash</title>
      <genre>Romance</genre>
      <number>1247</number>
   </book>
   <book>
      <author>Thurman, Paula</author>
      <title>Splish Splash</title>
      <genre>Romance</genre>
      <number>1248</number>
   </book>
   <book>
      <author>Knorr, Stefan</author>
      <title>Creepy Crawlies</title>
      <genre>Horror</genre>
      <number>1241</number>
   </book>
   <book>
      <author>Kress, Peter</author>
      <title>Paradox Lost</title>
      <genre>Science Fiction</genre>
      <number>1242</number>
   </book>
   <book>
      <author>O'Brien, Tim</author>
      <title>Microsoft .NET: The Programming Bible</title>
      <genre>Computer</genre>
      <number>1243</number>
   </book>
   <book>
      <author>O'Brien, Tim</author>
      <title>MSXML3: A Comprehensive Guide</title>
      <genre>Computer</genre>
      <number>1244</number>
   </book>
   <book>
      <author>Galos, Mike</author>
      <title>Visual Studio 7: A Comprehensive Guide</title>
      <genre>Computer</genre>
     <number>1245</number>
   </book>
</catalog>

Here is the expected output:

<?xml version="1.0"?>
<catalog>
   <book>
      <author>Gambardella, Matthew</author>
      <title>XML Developer's Guide</title>
      <genre>Computer</genre>
      <number>1234</number>
   </book>
   <book>
      <author>Ralls, Kim</author>
      <title>Midnight Rain</title>
      <genre>Fantasy</genre>
      <number>1235</number>
   </book>
   <book>
      <author>Corets, Eva</author>
      <title>Maeve Ascendant</title>
      <genre>Fantasy</genre>
      <number>1236</number>
   </book>
   <book>
      <author>Corets, Eva</author>
      <title>Oberon's Legacy</title>
      <genre>Fantasy</genre>
      <number>1237</number>
   </book>
   <book>
      <author>Corets, Eva</author>
      <title>The Sundered Grail</title>
      <genre>Fantasy</genre>
      <number>1238</number>
   </book>
   <book>
      <author>Randall, Cynthia</author>
      <title>Lover Birds</title>
      <genre>Romance</genre>
      <number>1239</number>
   </book>
   <book>
      <author>Thurman, Paula</author>
      <title>Splish Splash</title>
      <genre>Romance</genre>
      <number>1248</number>
   </book>
   <book>
      <author>Knorr, Stefan</author>
      <title>Creepy Crawlies</title>
      <genre>Horror</genre>
      <number>1241</number>
   </book>
   <book>
      <author>Kress, Peter</author>
      <title>Paradox Lost</title>
      <genre>Science Fiction</genre>
      <number>1242</number>
   </book>
   <book>
      <author>O'Brien, Tim</author>
      <title>Microsoft .NET: The Programming Bible</title>
      <genre>Computer</genre>
      <number>1243</number>
   </book>
   <book>
      <author>O'Brien, Tim</author>
      <title>MSXML3: A Comprehensive Guide</title>
      <genre>Computer</genre>
      <number>1244</number>
   </book>
   <book>
      <author>Galos, Mike</author>
      <title>Visual Studio 7: A Comprehensive Guide</title>
      <genre>Computer</genre>
     <number>1245</number>
   </book>
</catalog>

Where the author is Thurman, Paula. The largest number of the node should be retrieved. Is there any way that I can retrieve the output? If so, please guide me in the right direction.

Thanks in advance!!

Answer 1

You can group your books by a string that comprises all duplicate values (author, title, genre) and return the book in that group which has the maximum number:

element catalog {
  for $book-group in catalog/book
  group by $key := string-join($book-group/(author, title, genre), '|')
  let $max-number := max($book-group/number)
  return $book-group[number = $max-number]
}