If the oldvalue is nullthe result of the equations should rightly be null. This is becasue null is not a number, it means unknown value. Therefore the calcaluation cannot proceed.

Of course your equation also fails if oldvalue = 0 as you cannot divide by zero.

Typically you handle these things with a case statment that will take you the path of what you want to show. So if you want the result to show as null when you can't calculate (as with a null or a zero, then you would do a case statment something like

case when [oldvalue] = 0 then null else 100 * (([old value] - [new value]) /[old value]) end 

If the old value is null the result of the equations should rightly be null. This is because null is not a number, it means unknown value. Therefore the calculation cannot proceed.

Of course your equation also fails if old value = 0 as you cannot divide by zero.

Typically you handle these things with a case statement that will take you the path of what you want to show. So if you want the result to show as null when you can't calculate (as with a null or a zero, then you would do a case statement something like

case when [old value] = 0 then null else 100 * (([old value] - [new value]) /[old value]) end