hide Drop down list if no need

[garevn]

Hello i am really new in programming and i cant figure out how to solve the following problem.
i am having a drop down list for the user helping him reduce results.

My list looks like this

<select name="products2" id="soch" onchange="showUser()">
           <option value="" selected="selected">Sochet</option>
           <?php
do {  
?>
           <option value="<?php echo $row_sochet['sochet']?>"><?php echo $row_sochet['sochet']?></option>
           <?php
} while ($row_sochet = mysql_fetch_assoc($sochet));
  $rows = mysql_num_rows($sochet);
  if($rows > 0) {
      mysql_data_seek($sochet, 0);
      $row_sochet = mysql_fetch_assoc($sochet);
  }
?>
         </select>

for example when the user choose the processors category then possible sochets display.

I want to hide this drop down if he choose another category ex. printers which dont contain any sochet