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

Performance is a incredible better when using CASE statement:

SELECT ProductID, ProductName,ProductDesc 
FROM product 
WHERE ProductID = CASE WHEN @productID IS NULL THEN ProductID ELSE @productID END

ISNULL() prevents the optimizer using an index on that column.