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.