SQL select all if parameter is null else return specific item

Use case statement:

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

Or IIF() function if you’re using SQL Server 2012:

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

Try this

DECLARE @productID INT = NULL

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

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