esql does not handle to_date function in SQL

Hi, there.

Here is my xml source:

<sql:execute-query xmlns:sql="http://apache.org/cocoon/SQL/2.0";>
        <sql:use-connection>personnel</sql:use-connection>
        <sql:query>
           select count(pt)
            from adv_7760$rollsnap.thro 
            where study = 'ADV_7760'
            and vpos = 'aortic'
            and to_date(event_date,'yyyymmdd') - 
to_date(implant_date,'yyyymmdd') <= 30 
            and thrombosed_etiology = 'primary event'
        </sql:query>
</sql:execute-query>

I am using cocoon to transform the results into xml.  My problem is cocoon is 
confused with to_date function.  It cannot process the query.  If I take the 
line with the to_date function, it works.  Is there any way I can accomplish 
the query in other ways?

Thanks in advance for your help.

Narisa Ibeling
Medtronic Cardiac Surgery



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