I know I am going about this in an unusual way, every tut I've seen uses multiple tables, but due to the way the rest of my site works I would like to create a chained select which operates using a single table.

My table structure is:

----------------------
|Catagory|SubCategory|
|01|cat1 |subcat1    |
|02|cat1 |subcat2    |
|03|cat2 |subcat1    |
|04|cat2 |subcat2    |
----------------------

The code I have so far looks like:

<tr>
    <td class="shadow"><strong>Category:</strong>   </td>
    <td class="shadow">
    <select id="category" name="category" style="width:150px">
        <option selected="selected" value="<?php echo $category ?>"><?php echo $category?></option>
        <?php
        include('connect.php');
        $result1 = mysql_query("SELECT DISTINCT category FROM categories")  
            or die(mysql_error());  

            while($row = mysql_fetch_array( $result1 )) {
            $category = $row['category'];
            echo "<option value='". $row['category'] ."'>". $row['category'] ."</option>";
            }
        ?>        
    </select>
   </td>
</tr>
<tr>
    <td class="shadow"><strong>Sub Category:</strong>   </td>
    <td class="shadow">
    <select id="sub_catgory" name="sub_category" style="width:150px;">
        <option selected="selected" value="<?php echo $sub_category ?>"><?php echo $sub_category ?></option>
        <?php
            include('connect.php');
            $result2 = mysql_query("SELECT sub_category FROM categories WHERE ")
                or die(mysql_error());

                while($row = mysql_fetch_array ($result2 )){
                echo "<option value='" . $row['sub_category'] . "'>". $row['sub_category']. "</option>";
        }

        ?>    
    </select>
    </td>           
</tr>

On the second select I am not sure how to state the WHERE clause. I need it to display the subcategories which have the same category as selected in the first select.

PART 2 how would I include AJAX in this to preload the data so i don't need to refresh the page.

Could someone either help me finish what I've started here or point me to a good tutorial.

thanks