protected function sql($sql = null, $replace = false)
    {
        if (null !== $sql) {
            if (!isset($this->oosql_sql[$this->oosql_table]) || $replace) {
                $this->oosql_sql[$this->oosql_table] = $sql;
            } else {

                $this->oosql_sql[$this->oosql_table] .= $sql;
            }
            return;
        }
        return $this->oosql_sql[$this->oosql_table];
    }