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> ([/basics/catalog.xml 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. [/basics/FLWOR.xq 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> [/basics/FLWOR-1.xq 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> [/basics/FLWOR-2.xq 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.