I find the variable assignment rather interesting in that it seems to occur at a curious point. To see what happened when there is a null value, I tried:

declare @txt varchar(255)

set @txt = 'Question'

insert xxx values ('')

select @txt = isnull (a, '?') from xxx order by i

and got a null string and not "?" as I was expecting.

Does you know why? It seems to me that the variable assignment occurs at a rather odd place in the execution of the statement.