sql to filter data from dropdown boxes

[eluser]max123[/eluser]
I have 2 dropdown boxes called country and category. Default value for Both text boxes is 'All'. I have a problem with writing the query.

If a country is selected(eg:USA) but All from category then USA results should display without considering the category. If a category is selected (eg: phone) but All from country then results about phone should display without considering the country

How can do this. How can I write the sql query

Thanx