title: SQL select all if parameter is null else return specific item
body: <p>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 null. 
  What I have so far:</p>
  
  <pre><code>DECLARE @productID INT = NULL
  
  SELECT
      ProductID,
      ProductName,
      ProductDesc 
  FROM
      product 
  WHERE
      ProductID = @productID
  </code></pre>