Database INSERT changes default 'no' to NULL

Hi

First of all you need to check checkbox - checked or not like below:
if (isset($_POST['guidelines_signed']) && $_POST['guidelines_signed'] == 'on')
{
//if checked than append value in insert array
$client_data['guidelines_signed'] = $this->input->post('guidelines_signed');
}
else
{
//if uncheked skip to add check box value
}
and also set null default value for "guidelines_signed" column in database table