public function getModifyTablePrimaryKeyDDL(TableDiff $tableDiff)
    {
        $ret = '';

        if ($tableDiff->hasModifiedPk()) {
            $ret .= $this->getDropPrimaryKeyDDL($tableDiff->getFromTable());
            $ret .= $this->getAddPrimaryKeyDDL($tableDiff->getToTable());
        }

        return $ret;
    }