count all result from a table using where clause

[eluser]ibnclaudius[/eluser]
I know how to do it using num_rows(), but how can I do this using count_all()?

I tried this, but no success:

Code:

public function count_school_update_comments($update_id)
{
  $query = $this->db->count_all($this->school_updates_comments_table)
    ->where("update_id", $update_id);

  return $query;
}