XQuery - If Then Else

XQuery provides a very useful if-then-else construct to check the validity of the input values passed. Given below is the syntax of the if-then-else construct.

Syntax

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

Example

We will use the following books.xml file and apply to it XQuery expression containing an if-then-else construct to retrieve the titles of those books with a price value that is greater than 30.

books.xml

<?xml version="1.0" encoding="UTF-8"?>
<books>
   <book category="JAVA">
      <title lang="en">Learn Java in 24 Hours</title>
      <author>Robert</author>
      <year>2005</year>
      <price>30.00</price>
   </book>
   
   <book category="DOTNET">
      <title lang="en">Learn .Net in 24 hours</title>
      <author>Peter</author>
      <year>2011</year>
      <price>40.50</price>
   </book>
   
   <book category="XML">
      <title lang="en">Learn XQuery in 24 hours</title>
      <author>Robert</author>
      <author>Peter</author>
      <year>2013</year>
      <price>50.00</price>
   </book>
   
   <book category="XML">
      <title lang="en">Learn XPath in 24 hours</title>
      <author>Jay Ban</author>
      <year>2010</year>
      <price>16.50</price>
   </book>
</books>

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

books.xqy

<result>
{
   if(not(doc("books.xml"))) then (
      <error>
         <message>books.xml does not exist</message>
      </error>
   )
   else ( 
      for $x in doc("books.xml")/books/book	
      where $x/price>30
      return $x/title
   )
}
</result>

Output

<result>
   <title lang="en">Learn .Net in 24 hours</title>
   <title lang="en">Learn XQuery in 24 hours</title>
</result>

Verify the Result

To verify the result, replace the contents of books.xqy (given in the Environment Setup chapter) with the above XQuery expression and execute the XQueryTester java program.