Dynamic Dependent Select Box using Jquery and Ajax

How to do dynamic dependent select box using Jquery, Ajax, PHP and Mysql. Dependent select box when a selection is made in a "Parent" box it allow to refresh a "child" box list data. In this post I had given a database relationship example betweent "catergory" and "subcategory". It's very simple jquery code hope you like this. 

 Download Script     Live Demo

Database
Sample database tables. Data table contains list boxes complete data, data_parenttable foreign key relationship with Data table contains parent and child relation.

CREATE TABLE 'data'
(
'id' int primary key auto_increment,
'data' varchar(50),
'weight' int(2),
);

CREATE TABLE `data_parent`
(
`pid` int(11) primary key auto_increment,
`did` int(11) unique,
`parent` int(11),
Foreign key(did) references data(id)
)

Data storing like this.
data data_parent


sections_demo.php
Contains javascipt and PHP code. $(".country").change(function(){}- country is the class name of select box. Using $(this).val() calling select box value. PHP code displaying results from data table where weight='1'

<script type="text/javascript" src="http://ajax.googleapis.com/
ajax/libs/jquery/1.4.2/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function()
{
$(".country").change(function()
{
var id=$(this).val();
var dataString = 'id='+ id;

$.ajax
({
type: "POST",
url: "ajax_city.php",
data: dataString,
cache: false,
success: function(html)
{
$(".city").html(html);
}
});

});

});
</script>
//HTML Code
Country :
<select name="country" class="country">
<option selected="selected">--Select Country--</option>
<?php
include('db.php');
$sql=mysql_query("select id,data from data where weight='1'");
while($row=mysql_fetch_array($sql))
{
$id=$row['id'];
$data=$row['data'];
echo '<option value="'.$id.'">'.$data.'</option>';
} ?>
</select> <br/><br/>

City :
<select name="city" class="city">
<option selected="selected">--Select City--</option>
</select>

ajax_city.php
Contains PHP code. Displaying results from data and date_parent tables

<?php
include('db.php');
if($_POST['id'])
{
$id=$_POST['id'];
$sql=mysql_query("select b.id,b.data from data_parent a,data b where b.id=a.did and parent='$id'");
while($row=mysql_fetch_array($sql))
{
$id=$row['id'];
$data=$row['data'];
echo '<option value="'.$id.'">'.$data.'</option>';
}
}
?>