[sql] CASE in WHERE, SQL Server

I'm doing some search, where users are choosing in dropdown some clauses. When they leave some box empty, I want query to ignore clause. I know CASE, and best I thought of is that if I pass 0 to parameter in stored procedure, it ignores that parameter, like this.

WHERE a.Country = (CASE WHEN @Country > 0 THEN @Country ELSE (something else) END)

so, (something else) should be like no condition, it can be '>0' as country ids are from all >1, but I don't know how to use > and = in same CASE.

Any suggestions?

(something else) should be a.Country

if Country is nullable then make(something else) be a.Country OR a.Country is NULL

.. ELSE a.Country ...

I suppose

Try this:

WHERE a.Country = (CASE WHEN @Country > 0 THEN @Country ELSE a.Country END)

You can simplify to:

WHERE a.Country = COALESCE(NULLIF(@Country,0), a.Country);