tabel rw(id_rw,nama_rw)
ax1  RW1
ax2  RW2

tabel rt(id_rt,id_rw,nama_rt)
az1 ax1 RT1
az2 ax2 RT4
az3 ax1 RT5

<?php
mysql_connect("localhost","root","");
mysql_select_db("coba");
$rw =mysql_query("select * from rw");
$resultx = array();
    while ($baca_data_rw = mysql_fetch_array($rw)) {
        $rt = mysql_query("select * from rt where id_rw = '$baca_data_rw[id_rw]' ")or die(mysql_error());
        $jump = mysql_num_rows($rt);
          echo "<h2>nama : $baca_data_rw[nama_rw] | id : $baca_data_rw[id_rw] | jumlah rt: $jump</h2>";
              $r = 1;
              while ($baca_data_rt = mysql_fetch_array($rt)) {
              echo $baca_data_rt['nama_rt']." | ".$baca_data_rt['id_rw']."</br>";
                  $resultx[]=$baca_data_rt['id_rw'];
              }
              echo count($resultx);
              echo "<pre>";
        print_r($resultx);
    echo"</pre>";

      }
?>
output array :
pertama
Array
(
    [0] => ax1
    [1] => ax1
)
kedua
Array
(
    [0] => ax1
    [1] => ax1
    [2] => ax2
)

Harusnya yg kedua cuma
Array
(
    [0] => ax2
)