create table tgt ( id, val ) as
select 1, 'a' from dual union all
select 2, 'b' from dual
;
Table TGT created.
create table src ( id, val ) as
select 1, 'x' from dual union all
select 2, 'y' from dual
;
Table SRC created.
update
( select t.val as t_val, s.val as s_val
...
