My simple and quick solution is the use of a normal $wpdb->query() function:
$wpdb->query( $wpdb->prepare("UPDATE $schedule_table SET rpt = %s WHERE post_id = %d AND start IS NULL AND end IS NULL", $event_tag, $post_ID ) );
My simple and quick solution is the use of a normal $wpdb->query() function:
$wpdb->query( $wpdb->prepare("UPDATE $schedule_table SET rpt = %s WHERE post_id = %d AND start IS NULL AND end IS NULL", $event_tag, $post_ID ) );