How to load second dropdown list frommongodb collection after first dropdown list is changed

I have floor collection in mongodb. floor have name and room no. i was write a php query within html drop down box for list out the floor name .query is

<select class="select">

  <?php

$collection = $db->floors;

$cursor = $collection->find();

  if($cursor){ 

  foreach($cursor as $rec){

echo"\n";?>

<option>  <?php echo $rec['name']; ?> </option>

<?php }

                                         }

    ?> 

  </select>

now i want select room list based ob floor name

this html form is display the room no list

<label class="col-sm-4 control-label">Room Number</label>

<div class="col-sm-8">

<select class="select">

<option>01</option>

<option>02</option>

<option>03</option>

<option>04</option>

</select>

</div>

how can i write a query for selection room no . pls anyone help me.advancein thanks

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