PHP variable para consultar en la BD

[ocesitaro]

<?
if($_POST){
if(!empty($_POST['vereda'])){
$tabla=$_POST['vereda'];
}else{
if(!empty($_POST['sector'])){
$tabla=$_POST['sector'];
}
}else{
echo "<script>alert('selecciona algo');</script>";
}
}

$res = mysql_query("SELECT $_POST[item] as item, count($_POST[item]) as total FROM $tabla GROUP BY $_POST[item]", $conexion);
$num_candidates = mysql_num_rows($res);
?>