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

Since "" is not recognized as NULL I used value

SELECT ProductID, ProductName,ProductDesc 
FROM product 
WHERE ProductID =IIF(@productID =1, ProductID, @productID )

In my code:

 MyDataAdapter.SelectCommand.Parameters("@productID").Value = 1