Full outer join to show differences

Hi,

I'm comparing 2 tables after a test with a full outer join ie

select * from before b full outer join after a on b.id=a.id

But I only want to see the data that's different in the after table or not there at all, ie null in that select.

So I add
where a.data<>b.data

This now excludes the missing after lines.

So I need something like

select * from before b full outer join after a on b.id=a.id
where a.data<>b.data or
b.data= null

How do I get those null rows?