$public_prod_id = 'C'.rand(1000000000,9999999999);
    $_SESSION['public_prod_id'] = $public_prod_id;

 $exist = mysqli_num_rows($pdo->query(
      "SELECT * FROM `product`
      WHERE `public_prod_id` = '{$_SESSION['public_prod_id']}' "
    ));

if($exist == 0){
$pdo->query(
      "INSERT INTO ``
      (
        ``
      )
      VALUES
      (
        '{}'
      )
      "
    );
 }

如果找到的結果是大於0
那我如何回到上面再產生一組新的號碼，然後再重新走一次？