function values(array $values)
    {
        $cols = array();
        $vals = array();

        foreach ($values as $col => $val) {
            if (null !== $val) {
                $cols[] = $col;
                $vals[] = $val;
            }
        }

        foreach ($cols as &$col) {
            $this->nodes->columns[] = new UnqualifiedColumn($col);
        }

        $this->nodes->values = array_merge($this->nodes->values, $vals);
        return $this;
    }