SQL select all if parameter is null else return specific item

Question

Is there a way to write the following script so that it returns all products if the ProductID variable is null ? And return a specific product when the product it is not nul

Answer 1

Why not just:

DECLARE @productID INT = NULL

SELECT ProductID, ProductName,ProductDesc 
FROM   product 
WHERE  ProductID = @productID
OR     @productID IS NULL;

Here a demo in SQLFiddle with NULL and a value for @productID