aki te dejo como esta hecho ahora mismo arriba tienes la funcion ver, si kieres ver la base de tatos te mando una prueba, gracias
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style type="text/css">
.ocultar {display:none;}
.mostrar {display:block;}
.sel {color:#FF0000;}
.nosel {color:#000000;}
</style>
<script type="text/javascript">
var visto_subm = null;
var visto_color = null;
function ver(num) {
objhl = document.getElementById(num);
obj = document.getElementById("sub"+num);
obj.className = (obj==visto_subm) ? 'ocultar' : 'mostar';
objhl.className = (objhl==visto_color) ? 'nosel' : 'sel';
if ((visto_subm != null) && (visto_color != null)) {
visto_subm.className = 'ocultar';
visto_color.className = 'nosel';
}
visto_subm = (obj==visto_subm) ? null : obj;
visto_color = (obj==visto_color) ? null : objhl;
}
</script>
</head>
<body>
<div id="sectores">
<label class="subcabezeras">Actividad de la empresa</label>
<?php
$link = mysql_connect($host,$usuario,$password);
mysql_select_db($bd,$link) or die (mysql_error());
$result = mysql_query("SELECT * FROM sectores", $link);
$row = is_resource($result) ? mysql_num_rows($result) : 0;
$row = mysql_fetch_array($result);
$num_rows = mysql_num_rows($result);
echo "<table border = '0'> \n";
do {
$id = $row["id_sector"]-1;
if ( $id%5==0 ){
echo "<tr>";
$count=0;
$count=$count+1;
echo "<td>";
}else{
echo "<td>";
}
echo "<th>";
echo "<div id=".$id." class=\"nosel\" onclick=\"ver(this.id)\" style=\"z-index:1;\">".htmlentities($row["nom_sector"])."</div>";
echo "</th>";
if ( ($id%5!=0) && ($count<5) ){
echo "</td>";
}
if($count==5){
echo "</tr>";
}
echo "<div id=\"sub".$id."\" class=\"ocultar\" style=\"z-index:1;\">";
//conusulta tabla subsectores
$result_sub = mysql_query("SELECT * FROM subsectores where id_sector=\"$row[id_sector]\" ", $link);
do{
//$row_sub = is_resource($result_sub) ? mysql_num_rows($result_sub) : 0;
$row_sub= mysql_fetch_array($result_sub);
$num_rows_sub = mysql_num_rows($result_sub);
echo "<input name=\"check_sub[]\" id=\"check_sub\" type=\"checkbox\" value=".$row_sub["id_subsector"]." /><label>".htmlentities($row_sub["nom_subsector"])."</label>";
}while ($row_sub = mysql_fetch_array($result_sub));
echo "</div>";
} while ($row = mysql_fetch_array($result));
echo "</table>";
//mysql_free_result($result);
?>
</body>
</html>