Dependant Dropdown Menus

[powerdrive]

It's probably only related to osC peripherally; I'm trying to teach myself a bit more php/mysql so that I can use it on my site.

I've set up a bit of space on the web server and the db server so that I can play about with stuff and I'm close to doing what I want to but I've hit an impasse.

 

I have a drop down menu populated with three categories from one table in my db and another for subcategories of which there are five for each main category.

 

//categories drop down
$result = mysql_query ("SELECT * FROM dependant_dropdown_test_parent");
echo "<h1>Test</h1><table><tr><td>";
echo "Categories</br><select>Categories";
echo "<option value='0'>Select a category</option>";
while ($row = mysql_fetch_array ($result))
 {
 echo "<option value=" .$row['parent_id']. ">" . $row['parent_value']. "</option>";
 }
echo "</select>";
echo "</td><td>";

//sub-categories drop down
$result = mysql_query ("SELECT * FROM dependant_dropdown_test_child");
echo "Sub-categories</br><select>Sub-categories";
echo "<option value='0'>Select a sub-category</option>";
while ($row = mysql_fetch_array ($result))
 {
 echo "<option value=" .$row['child_id']. ">" .$row['child_value']. "</option>";
 }
echo "</select></td></tr></table>";

 

I want to take the selected "parent_id" from the first drop down and modify the second query so that it ends with "WHEN parent_id = $...".

I also want to hide the second one until a selection has been made on the first (or if there was a category with no subcategories), but I'm happy to take it one step at a time.

I appreciate any help.

Regards,

Power Drive