combo box fill aginst data in textbox

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PHP

MySQL

i have fill data in combo box list, after i select combo box list fill text box.
for example in combo box i select city then fill location how can i do ? how can i wirte second query in where clause ?

combo box code

<select name="cmbJob" id="SearchItem1" class="cmbJob" onChange=" document.getElementById('txtjob').value=this.value;   document.getElementById('JOB_NO').value=this.options[this.selectedIndex].text; document.getElementById('idValue').value=this.options[this.selectedIndex].value;">
              <option value="0" selected="selected">---Select JobNo---</option>
              <?php
                  include('dbconnection.php');
                  $sql=mysql_query("select distinct JOB_NO from mm_client_loc_job");
                   while($row=mysql_fetch_array($sql))
                    {

                    $id=$row['JOB_NO'];

                    $data=$row['JOB_NO'];

                    //echo '<option value=>'.$data.'</option>';
                     echo '<option value="'.$id.'">'.$data.'</option>';
                     }
              ?>
</select>