How To Get Departments Based on Course

[eluser]prakash[/eluser]
Hi iam using ci form tags to design a form.....
Here i want to get departments based on the course selected so how to move on
Here is my code
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<tr class="fieldbox">
<td width="311" align="right" height="23">
&lt;?php echo form_label('Courses','course'); ?&gt;</td>
<td width="33"> <strong>:</strong> </td>
<td width="431" colspan="2">&lt;?php
$course = $this->db->query("select * from courses");
foreach($course->result() as $courses)
{
$coursenames[]= $courses->course_name;
}

echo form_dropdown('Courses', $coursenames, 'course');?&gt; </td>
</tr>

<tr class="fieldbox">
<td width="311" align="right" height="23">&lt;?php echo form_label('Department','dept'); ?&gt; </td>
<td width="33"> <strong>:</strong> </td>
<td width="431" colspan="2">&lt;?php
$dept = $this->db->query("select * from departments");
foreach($dept->result() as $depts)
{
$ddata[]= $depts->dept_name;
}

echo form_dropdown('departments', $ddata, 'dept');?&gt; </td>
</tr>