update empty column according to existen data

eventually I want to update en2_id according to en1 which mean if en2=en1 set en2_id to en1_id. If en2 is not in en1 then create a new id for en2_id

the end table looks like
en1_id | en1 | en2_id | en2

1        A     2        B
2        B     3        C
3        C     4        D 

I first try to build a view
>create view VT as select * from T;

then I do update
>update T

set en2_id=en1_id
where T.en2 in(select VT.en1 from VT);