Hi,
There could be several approaches.
It also depends on the way you have constructed your current query.
If the idea is to be able to filter some column on a selected value or not to filter at all, I would recommend the approach using the "ifNot" operator as described on page 88 of the documentation.
In order to be more specific, I would need some more information about your current query.
Are you using some other dataset for specifying the values or your parameters so that the user can select the value in a drop down list or are you just entering the value of the parameter in a text box?
Let me know.
Regards,