function IN($items)
    {
        $comma_separated_items = $this->JoinNotEmpty("','", is_array($items) ? $items : explode(',', $items));
        $count_items = substr_count($comma_separated_items, ',') + 1;

        if (trim($comma_separated_items) == '') {
            $count_items = 0;
        }

        if ($count_items > 1) {
            return " IN ('$comma_separated_items') ";
        } elseif ($count_items == 1) {
            return " = '$comma_separated_items' " ;
        } else {
            return ' IS NULL ' ;
        }
    }