select with parent and child categories

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HI.

i am working on a program that have categories and sup categories, the thing that the main category have a prent_id=0

and the sup category hase a the parent category_id

 

the probleme now that I wont to be able to update the parent_id and when the parent_id=0 it will show me an option says "no parent" with the value of 0

 

		<SELECT SIZE='1' name='parent_id'>";
$categories = mysql_query("SELECT * FROM categories where category_id<>$category_id ",$db);
while ($categories_data = mysql_fetch_array($categories))
{
$category_id=$categories_data['category_id'];
$category_title=$categories_data['category_title'];
   if($category_id == $parent_id)
   {
     echo"<OPTION SELECTED value='$category_id'>$category_title</option>";
   }
  else
   {
     echo"<option value='$category_id'>$category_title</option>";
   }
}
     echo"</select>

 

i have worked around it in the add section and am using this code

 

<SELECT SIZE='1' NAME='parent_id' dir='rtl' tabindex='1'>
<option value='0'>no parent</option>";

$categories = mysql_query("SELECT * FROM categories",$db);
while ($categories_data = mysql_fetch_array($categories))
{
$pcategory_id=$categories_data["category_id"];
$pcategory_title=$categories_data["category_title"];
echo"<option value='$pcategory_id'>$pcategory_title</option>";
}
echo"</SELECT>