The first one will always be checked, after that any ANDs will be "anded" with the first one and any ORs will be "orred" with the first one. You can make it more complex, but that will complicate the where statement.
Can you describe what you actually want?
I'm afraid I need more to go on. Do you have a user interface created for the search that will let you determine what kind of search it will be? You could post a copy of the form, and a description of what you want to happen based on the submitted results.
I struggled with a similar problem, as much as I like using xPDO, I finally broke down and just created an SQL query string that I used to query the table.
It was much easier to develop since I was able phpMyAdmin to build and test the query string. Once I had a query string that gave me the results I wanted, it was just a matter of writing code to plugin the values being sent by the form.
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