An example of how you can programmatically access an InfoPath secondary DataSource and use it requery with a new value.

//Construct a new command for the second dropdown listbox using a Where clause...

XDocument.DataObjects("SecondaryDataSourceName ").QueryAdapter.Command = originalSQLQuery1 +

            " WHERE Items='" + TestValue + "'"

//Requery the data source

XDocument.DataObjects("SecondaryDataSourceName").Query();  

//Force the view to refresh

XDocument.View.ForceUpdate();

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