[Zope] Question involving passing optional parameters to a zsql method

[Laura McCord]

I have a form with 7 checkboxes. The more checkboxes are selected the
more narrow of results you will get. I created a zsql method that will
handle every case as far as when to include 'AND','OR', and '()'. 

The problem now is getting my form to execute the zsql method correctly.
When I hit the submit button the boxes that are not check will trigger
an error message stating that the variable was not defined.

I am trying to figure out when I hit the submit button my dtml method
that runs the zsql method is not written to adjust for empty parameters.
I had originally called my function like this:
 <dtml-call expr="findResults(location1=location1, location2=location2,
..., usetype1=usetype1, usetype2=usetype2,...,active=active)">

Does anyone have an idea on how to call this function regardless of what
boxes are checked?

thanks.