This is a sightly different (and simpler IMHO) than this post:<br />
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http://www.birt-exchange.com/devshare/designing-birt-reports/688-using-a-multivalue-parameter-in-an-in-clause/#description<br />
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In my query I have something like this:
where PRODUCTLINE in ('****')
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and in my beforeOpen Script:
this.queryText = this.queryText.replace("****", params["pProductLines"].value.join("','" ));
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Note the separator of the join function - ',' (single quote, comma, single quote). Attached is a sample using this technique.