How can I remove duplicate nodes in XQuery?

Question

I have an XML document I generate on the fly, and I need a function to eliminate any duplicate nodes from it.

My function looks like:

declare function local:start2() {
    let $data := local:scan_books()
    return <books>{$data}</books>
};

Sample output is:

<books>
  <book>
    <title>XML in 24 hours</title>
    <author>Some Guy</author>  
  </book>
  <book>
    <title>XML in 24 hours</title>
    <author>Some Guy</author>  
  </book>
</books>

I want just the one entry in my books root tag, and there are other tags, like say pamphlet in there too that need to have duplicates removed. Any ideas?

---

Updated following comments. By unique nodes, I mean remove multiple occurrences of nodes that have the exact same content and structure.

Answer 1

A simpler and more direct one-liner XPath solution:

Just use the following XPath expression:

  /*/book
        [index-of(/*/book/title, 
                  title
                 )
                  [1]
        ]

When applied, for example, on the following XML document:

<books>
    <book>
        <title>XML in 24 hours</title>
        <author>Some Guy</author>
    </book>
    <book>
        <title>Food in Seattle</title>
        <author>Some Guy2</author>
    </book>
    <book>
        <title>XML in 24 hours</title>
        <author>Some Guy</author>
    </book>
    <book>
        <title>Food in Seattle</title>
        <author>Some Guy2</author>
    </book>
    <book>
        <title>How to solve XPAth Problems</title>
        <author>Me</author>
    </book>
</books>

the above XPath expression selects correctly the following nodes:

<book>
    <title>XML in 24 hours</title>
    <author>Some Guy</author>
</book>
<book>
    <title>Food in Seattle</title>
    <author>Some Guy2</author>
</book>
<book>
    <title>How to solve XPAth Problems</title>
    <author>Me</author>
</book>

The explanation is simple: For every book, select only one of its occurences -- such that its index in all-books is the same as the first index of its title in all-titles.