How can I use MySQL GROUP BY modifier WITH ROLLUP in Zend_Db_Select?

The SQL query in my web application(PHP/MySQL + Zend) uses a number of parameters to build the search query. However, the records fetched should be grouped by two columns to get sub totals. WITH ROLLUP seems to be a good solution to 开发者_C百科find subtotals, but Zend_Db_Select doesn't support this modifier. Is there any work around?

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Well,

$select->from(array('t1' => 'table1'))
    ->where("where");
    ->group("(field1)  WITH ROLLUP");