wpdb num_rows总是返回0

Question

I have a form created with gravity forms (wordpress) and taking as input the "card number" and the email I want to see if the data associated with a user. I tried to make me return the number of rows in the query, but num_rows always returns 0. The "card number" is in the table usermeta.It copies the function code created.

function ricerca_fc_from_db($entry, $form) {
    global $wpdb;

    $wpdb->get_results("SELECT meta_value
          FROM $wpdb->usermeta wpusermeta
          FULL JOIN $wpdb->users wpusers ON (wpusermeta.user_id = wpusers.ID)
          WHERE wpusermeta.meta_value =".$entry[1]."
          AND  wpusers.user_email ='".$entry[2]."'
          AND wpusermeta.user_id = wpusers.ID");

    $row = $wpdb->num_rows;
    if($row == 0) {
    } else {
        header("location: http://stackoverflow.com/");
    }
}