public function addRow($data = null)
    {
        // Clear existing empty row
        if (($current = $this->currentRow()) && $current->isEmpty()) {
            array_pop($this->rows);
        }

        // Append data
        if ($data instanceof SQLAssignmentRow) {
            $this->rows[] = $data;
        } else {
            $this->rows[] = new SQLAssignmentRow($data);
        }
        return $this;
    }