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XQuery If Then Else Statement

How to use XQuery's if-then-else construct to validate input values and conditionally process XML data, with practical examples for filtering book prices.

XQuery If Then Else Statement

XQuery provides a very useful if-then-else construct for checking the validity of passed input values.

Syntax

if (condition) then
	...
else
	...

Example

Consider the following example that contains information about a bookstore.

books.xmlxml
<?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>

We want to use the if-then-else statement to retrieve the titles of those books with a price value that is greater than 20.

The following XQuery expression is to be applied on the above XML document.

books.xqyxquery
<result>
{
   if(not(doc("books.xml"))) then (
      <error>
         <message>books.xml does not exist</message>
      </error>
   )
   else ( 
      for $x in doc("books.xml")/bookstore/book	
      where $x/price>20
      return $x/title
   )
}
</result>

The XQuery expression above will generate the following result:

<result>
   <title lang="en">Harry Potter</title>
   <title lang="en">Learn XML</title>
</result>
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