Сравнить 2 даты из базы

$dej_vrach = mysql_query("SELECT duty.id_doctor as d_id_doc, DATE_FORMAT(duty.date, '%d %M %Y') as d_date, duty.time_n as d_time_n, duty.time_k as d_time_k,doctor.id_doctor as doc_id, doctor.first as first,doctor.last as last, doctor.middle as middle,doctor.special as doc_special,special.special as sp_special, special.spec_name as spec_name FROM duty, doctor, special WHERE doctor.id_doctor = duty.id_doctor AND doctor.special = special.special",$db);
 
if (mysql_num_rows($dej_vrach) > 0)
{
$mass_dej_vrach = mysql_fetch_array($dej_vrach);
}

$date_dafault = mysql_query("SELECT id, DATE_FORMAT(date_default, '%d %M %Y') as def FROM date_dafault WHERE id='1'");
 
if (mysql_num_rows($date_dafault) > 0)
{
$dafault = mysql_fetch_array($date_dafault);
}

if ($mass_dej_vrach['d_date'] < $dafault['def'])
{echo "Ошибка";}