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XQuery FLOWR

What is FLWOR?​

FLWOR is an acronym that stands for "For, Let, Where, Order by, Return".

The following list shows what they account for in a FLWOR expression:

  • For: selects a sequence of nodes
  • Let: binds a sequence to a variable
  • Where: filters the nodes
  • Order by: sorts the nodes
  • Return: what to return (gets evaluated once for every node)

Example​

Consider the following example that contains information about a bookstore.

We will use a FLOWR expression to retrieve the authors of these books with a price lower than 20.

books.xml
<?xml version="1.0" encoding="UTF-8"?>

<bookstore>
	<book category="children">
		<title lang="en">Harry Potter</title>
		<author>J K. Rowling</author>
		<year>2005</year>
		<price>29.99</price>
	</book>
	<book category="web">
		<title lang="en">XQuery Tips</title>
		<author>Tom Nolan</author>
		<year>2022</year>
		<price>39.99</price>
	</book>
	<book category="web">
		<title lang="en">Learn XML</title>
		<author>Tutorial Reference</author>
		<year>2022</year>
		<price>9.99</price>
	</book>
</bookstore>

And the following XQuery document contains the query expression to be executed on the above XML document:

books.xqy
let $books := (doc("books.xml")/bookstore/book)
return <results>
{
   for $x in $books
   where $x/price<20
   order by $x/author
   return $x/author
}
</results>

More in details:

  • The for clause selects all book elements under the bookstore element into a variable called $x.
  • The where clause selects only book elements with a price element with a value lower than 20.
  • The order by clause defines the sort-order. Will be sort by the author element.
  • The return clause specifies what should be returned. Here it returns the title elements.

The result of the XQuery expression above is:

<author>Tutorial Reference</author>

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