how to using javascript to create two level drop down list

hi all,
i want to create two level drop down list , and the data are retrive from mysql ,

the first and the second drop down list data are all from the same table of my db,
for example :
data are a, b,c,d
1.when user select 'a' from select_1 then select_ 2 only shown b,c,d
2.when user select 'b' from select_1 then select_ 2 only shown a,c,d
3......
anyone knows how to do ?

 

first write the html markup for them, and add onchange in 1st drop down like this

<select onchange="javascript:FunctionToCallServerBack(withSelectedItem) ..........

then do the DB process and re-render the second dropdown

 

it's my code , but still can't show drop down list for select_2 , what's wrong with my code ?

<script>
function buildSelect2($select1)
{
<?
echo "<td>";
$sql = "select addr from customer where addr <>$select1";
echo "<select name=\"select2\" size=\"0\" >" ;
for ($i=0;$i<$result_num;$i++)
{
$row = mysql_fetch_array($result);
$addr= $row['addr'];
echo "<option value=$addr>$addr</option>";

}
echo "</td>";
?>

}
</script>
..................
echo "<td>";
echo "<select name=\"select1\" size=\"0\" onchange = buildSelect2(this) >" ;
for ($i=0;$i<$result_num;$i++)
{
$row = mysql_fetch_array($result);
$addr= $row['addr'];
echo "<option value=$addr>$addr</option>";

}
echo "</td>";