SQL & PL/SQL

I'd like to update all the rows with values getting from subquery from the same table.
For example, the subquery of

select id, some_manipulation as value from table_a
id value
-------------------
1 12
2 14
3 16
4 21

I'd like to use above info update the values of a column (column2 of table_a) according to the same id. Something like this

update table_a a
set a.column2 = b.value
where a.id =
(select b.id from table_a b);

I know above sql sytax is invalid, but I just want to give you an idea what I'm tryig to accomplish. Any idea?

Thanks in advance,
Ittichai