Allowing parameter choice on dashboard from multiple existing parameters

Is it possible to create a query or a dashboard with a few parameters in the WHERE clause and then allow the user to choose on which of them to filter the query/dashboard? This means some of the parameters should be left empty/inactive and one or some will stay active and define the limits of the query.
i.e:

*SELECT *
x,y,z
*FROM *
table
*WHERE *
a = {{‘a’}}
*OR *
b = {{‘b’}}
OR
c = {{‘c’}}

I want the user to be able to filter the query/dashboard by parameter ‘a’ only, or ‘b’ only, or ‘c’ only.

Thank you.

Optional query parameters are not supported. But you can use the workaround I wrote here to create a similar effect. The parameters not in use would be set to 'All' or something similar.