XQuery FLWOR - Rookie Nerd

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XQuery FLWOR

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

For - It is used to select a sequence of nodes.
Let - It is used to bind a sequence to a variable.
Where - It is used to filter the nodes.
Order by - It is used to sort the nodes.
Return - It is used to specify what to return (gets evaluated once for every node).

XQuery FLWOR Example

Let's take an XML document having the information on the collection of courses. We will use a FLWOR expression to retrieve the titles of those courses which fees are greater than 2000.

courses.xml

snippet

<?xml version="1.0" encoding="UTF-8"?>
<courses>   
   <course category="JAVA">
      <title lang="en">Learn Java in 3 Months.</title>
      <trainer>Sonoo Jaiswal</trainer>
      <year>2008</year>
      <fees>10000.00</fees>
   </course>  
    <course category="Dot Net">
      <title lang="en">Learn Dot Net in 3 Months.</title>
      <trainer>Vicky Kaushal</trainer>
      <year>2008</year>
      <fees>10000.00</fees>
   </course>
    <course category="C">
      <title lang="en">Learn C in 2 Months.</title>
      <trainer>Ramesh Kumar</trainer>
      <year>2014</year>
      <fees>3000.00</fees>
   </course>
    <course category="XML">
      <title lang="en">Learn XML in 2 Months.</title>
      <trainer>Ajeet Kumar</trainer>
      <year>2015</year>
      <fees>4000.00</fees>
   </course>  
</courses>

Let's take the Xquery document named "courses.xqy" that contains the query expression to be executed on the above XML document.

courses.xqy

snippet

let $courses := (doc("courses.xml")/courses/course)
return <results>
{
   for $x in $courses
   where $x/fees>2000
   order by $x/fees
   return $x/title
}
</results>

Create a Java based XQuery executor program to read the courses.xqy, passes it to the XQuery expression processor, and executes the expression. After that the result will be displayed.

XQueryTester.java

snippet

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;

import javax.xml.xquery.XQConnection;
import javax.xml.xquery.XQDataSource;
import javax.xml.xquery.XQException;
import javax.xml.xquery.XQPreparedExpression;
import javax.xml.xquery.XQResultSequence;

import com.saxonica.xqj.SaxonXQDataSource;

public class XQueryTester {
   public static void main(String[] args){
      try {
         execute();
      }
      
      catch (FileNotFoundException e) {
         e.printStackTrace();
      }
      
      catch (XQException e) {
         e.printStackTrace();
      }
   }

   private static void execute() throws FileNotFoundException, XQException{
      InputStream inputStream = new FileInputStream(new File("courses.xqy"));
      XQDataSource ds = new SaxonXQDataSource();
      XQConnection conn = ds.getConnection();
      XQPreparedExpression exp = conn.prepareExpression(inputStream);
      XQResultSequence result = exp.executeQuery();
       while (result.next()) {
         System.out.println(result.getItemAsString(null));
      }
   }	
}

Execute XQuery against XML

Put the above three files to a same location. We put them on desktop in a folder name XQuery3. Compile XQueryTester.java using console. You must have JDK 1.5 or later installed on your computer and classpaths are configured.

Compile:

javac XQueryTester.java

Execute:

java XQueryTester

Output: