Required Ajax Dropdown to fetch Categories & subcategories both from Database

[eluser]Unknown[/eluser]
Hi Guys,

I would be really thankful if you can tell me how to fetch categories and associated sub categories in drop down from database without Page Re-load (using ajax).

I have a categories drop down in which i have fetched all categories from table : categories

Now, what I want is that when a user selects a category, subcategories should be displayed in drop down in which all the subcategories would be coming from table against selected category id. I need to implement this in CodeIgniter.

PLEASE HELP!