OnChange Event - Populate Dropdown Fields Separately From Database

I have two drop-down fields that are populated separately from the database. Here is my HTML/PHP code
Drop-down field #1
<tr class=bodyTRstyle>
<td width="35%"><p align="left">Venue</td>
<td class=fieldTDstyle width="269"><p align="left">
<select name="VenueName" class=VenueName><option selected><?php echo $VenueName?></option

<?php
$SelectVenueQuery="select * from venue order by VenueName";
$SelectVenueResult=mysql_query($SelectVenueQuery);
while ($VenueRow=mysql_fetch_array($SelectVenueResult)){
extract($VenueRow);
echo "<option value='$VenueName'>$VenueName";}?>
</select></td></tr>

Drop-down Field #2
<tr class=bodyTRstyle>
<td width="35%"><p align="left">Results</td>
<td class=fieldTDstyle width="269"><p align="left">
<select name="GameResults" class=GameResults><option selected>
<?php echo $GameResults?></option
<?php
$ResultsQuery="select * from result order by ResultDate";
$ResultsQueryResult=mysql_query($ResultsQuery);

While ($ResultsRow=mysql_fetch_array($ResultsQueryResult)){
extract($ResultsRow);
echo "<option value='$ResultDate'>$ResultDate";}?>
</select></td></tr>

This code works well as long as the two drop-downs are independent of each other. The result table has VenueName as an element, so each venue has a set of result records in the result table. This is how I want to change it: The Venue dropdown will remain as it is. Once the Venue Name is selected I want the result dropdown to populate with ONLY the result records containing a match on VenueName that was selected in the venue dropdown. How do I do that? Is it with a onChange Event? What are the changes that need to be made? How do I communicate to the second drop-down what was selected in the Venue drop-down??