XQuery/FLWOR Expression

Motivation

You have a sequence of items and you want to create a report that contains these items.

Method

We will use a basic XQuery FLWOR expression to iterate through each of the items in a sequence. The five parts of a FLWOR expression are:

for - specifies what items in the sequence you want to select (optional)
let - used to create temporary names used in the return (optional)
where - limit items returned (optional)
order - change the order of the results (optional)
return - specify the structure of the data returned (required)

Lets assume we have a sample file of books that looks like this :

<books>
   <book>
      <title>Introduction to XQuery</title>
      <description>A beginner's guide to XQuery that covers sequences and FLOWR expressions</description>
      <type>softcover</type>
      <sales-count>155</sales-count>
      <price>19.95</price>
   </book>
  <book>
      <title>Document Transformations with XQuery</title>
      <description>How to transform complex documents like DocBook, TEI and DITA</description>
      <type>hardcover</type>
      <sales-count>105</sales-count>
      <price>59.95</price>
   </book>
   <!-- ...more books here.... -->
 </books>

(as stored in eXistdb)

Here is a simple example of a FLWOR expression that will return only the titles and prices of the books sorted by title:

for $book in doc("catalog.xml")/books/book
   let $title := $book/title/text()
   let $price := $book/price/text()
   where xs:decimal($price) gt 50.00
   order by $title
   return
      <book>
         <title>{$title}</title>
         <price>{$price}</price>
      </book>

This XQuery FLWOR expression will return all books that have a price over $50.00. Note that we have not just one but two let statements after the for loop. We also add a where clause to restrict the results to books over $50.00. The results are sorted by the title and the result is a new sequence of book items with both the price and the title in them.

Execute

Using the "to" function to generate a range of values

You can also express a range of values from one number to another number by placing the keyword "to" between two numbers in a sequence.

The following generates a list of values from 1 to 10.

xquery version "1.0";
<list>
   {for $i in (1 to 10)
      return
        <value>{$i}</value>
    }
</list>

Execute

Using the "at" function as a counter

You can add "at $my-counter" to add a numerical counter for each item in a FLWOR loop

xquery version "1.0";
<items>
{
let $items := ("apples","pears","oranges")
for $item at $count in $items
return
   <item id="{$count}">
      {$item}
   </item>
}
</items>

Execute

Comparing FLWOR with imperative for loops

In imperative programming languages (JavaScript, Java, .Net, Perl, PHP and Python) each loop is executed sequentially, one after another. However XQuery is a functional programming language. In a FLWOR loop, each iteration could execute in parallel and no communication is allowed between the threads. As a result, you cannot increment variables as you would in imperative languages.