CodeIgniter - return only one row?

You've just answered your own question :) You can do something like this:

$query = $this->db->get();
$ret = $query->row();
return $ret->campaign_id;

You can read more about it here: http://www.codeigniter.com/user_guide/database/results.html

This is better way as it gives you result in a single line:

$this->db->query("Your query")->row()->campaign_id;

To add on to what Alisson said you could check to see if a row is returned.

// Query stuff ...
$query = $this->db->get();

if ($query->num_rows() > 0)
{
    $row = $query->row(); 
    return $row->campaign_id;
}

return null; // or whatever value you want to return for no rows found

To make the code clear that you are intending to get the first row, CodeIgniter now allows you to use:

if ($query->num_rows() > 0) {
    return $query->first_row();
}

To retrieve the first row.