How to count occurences of a node in XML using XQuery?

Question

Input file:

<?xml version="1.0" encoding="UTF-8"?> 
        <books>
            <book id="6636551">
                <master_information>
                    <book_xref>
                        <xref type="Fiction" type_id="1">72771KAM3</xref>
                        <xref type="Non_Fiction" type_id="2">US72771KAM36</xref>
                    </book_xref>
                </master_information>
                <book_details>
                    <price>24.95</price>
                    <publish_date>2000-10-01</publish_date>
                    <description>An in-depth look at creating applications with XML.</description>
                </book_details>
            </book>
            <book id="119818569">
                <master_information>
                    <book_xref>
                        <xref type="Fiction" type_id="1">070185UL5</xref>
                        <xref type="Non_Fiction" type_id="2">US070185UL50</xref>
                    </book_xref>
                </master_information>
                <book_details>
                    <price>19.25</price>
                    <publish_date>2002-11-01</publish_date>
                    <description>A former architect battles corporate zombies, an evil sorceress, and her own childhood to become queen of the world.</description>
                </book_details>
            </book>
            <book id="119818568">
                <master_information>
                    <book_xref>
                        <xref type="Fiction" type_id="1">070185UK7</xref>
                        <xref type="Non_Fiction" type_id="2">US070185UK77</xref>
                    </book_xref>
                </master_information>
                <book_details>
                    <price>5.95</price>
                    <publish_date>2004-05-01</publish_date>
                    <description>After the collapse of a nanotechnology society in England, the young survivors lay the foundation for a new society.</description>
                </book_details>
            </book>
            <book id="119818567">
                <master_information>
                    <book_xref>
                        <xref type="Fiction" type_id="1">070185UJ0</xref>
                        <xref type="Non_Fiction" type_id="2">US070185UJ05</xref>
                    </book_xref>
                </master_information>
                <book_details>
                    <price>4.95</price>
                    <publish_date>2000-09-02</publish_date>
                    <description>When Carla meets Paul at an ornithology conference, tempers fly as feathers get ruffled.</description>
                </book_details>
            </book>
        </books>

I wrote XQuery to display the count of a specific field as follows:

for $x in //book_xref
let $c := string-join(('name of element:', count($x)), '
')
return $c

The intended output:

name of element: 4

but the output comes out as:

name of element:
1
name of element:
1
name of element:
1
name of element:
1

After which I understood why it is doing that. I tried aggregating the count value but that didn't work out. Also, couldn't find any function to fetch the name of the element automatically so that it's automatically included in the string.

Ideally, the target output is

book_xref:4

What do I need to achieve that? What am I missing?

Thanks! I appreciate your response.

Answer 1

How about just concat('book_xref:', count(//book_xref)) ?

The reason you're getting 4 different results in the output is because you're iterating over all occurrences of book_xref .

Also, you could've gotten the name by using $x/name() , but since you already know what you're selecting it's not necessary.

One easy, but not very efficient, way to get all element names with their number of occurrences would be:

let $names := distinct-values(//*/name())
for $x in $names
let $c := concat($x, ':', count(//*[name()=$x]), '
')
return $c

which produces:

books:1
book:4
master_information:4
book_xref:4
xref:8
book_details:4
price:4
publish_date:4
description:4

Answer 2

For your initial goal, @daniel-haley already has a concise solution.

If you want to efficiently count the number of occurrences of all element names in a document, in XQuery 3.0 you can use maps and the fn:fold-left(...) function to iteratively process all elements and keep counts that (at least in XQuery processors that support iterative evaluation) never have all elements in memory at the same time, not even the ones of the same name:

fold-left(
  //*,
  map{},
  function($map, $node) {
    let $name := $node/local-name()
    return map:merge((map { $name: ($map($name), 0)[1] + 1 }, $map))
  }
)

The simpler solution using group by is a lot more readable, but potentially less memory efficient:

map:merge(
  for $node in //*
  group by $name := $node/node-name()
  return map { $name: count($node) }
)

The result is the same for both queries:

map {
  'price': 4,
  'book': 4,
  'books': 1,
  'book_details': 4,
  'master_information': 4,
  'book_xref': 4,
  'xref': 8,
  'description': 4,
  'publish_date': 4
}