declare @a table (id int identity, s varchar(100));
declare @b table (id int identity, a_id int);

insert into @b
 (a_id)
 select
  m.id
 from
  (
   merge into @a t
   using (values (1, 'a')) s(id, s) on s.id = t.id
   when not matched then
    insert
     (s)
    values
     (s.s)
   when matched then
    update
     set s = s.s
   output
    inserted.id
  ) m(id);

select * from @a;
select * from @b;