How Can I Return Everything While Using A Paramater

I guess a pretty basic sql question here. I am using tableadapters. And some of the queries have multiple parameters. But often a parameter is at a default or null - wherein I'd like to be able to pass it in and have it not filter at all - ie., return everything.

Simple example:

Select * from Reports
Where ReportID = @ReportID 

 If I have a dropdownlist of reports where the values are ReportIDs - but the topmost unselected value is say "", then the results will be generated by

Select * from Reports
Where ReportID = ''

Whis not what I want. What I would want is simply

Select * from Reports

or

Select * from Reports
Where ReportID = ReportID

So how can one setup say an ObjectDataSource or somehow handle this where I don't need to use dynamic sql to expand/contract my where clause?

Really need hlpe on this and hope I've made my question reasonably clear. Thanks.