Update with use of subquery

Question

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

Answer 1

A slightly modified version of Sri's solution will work for all cases. Assuming proper indexing, this should be relatively fast.
SQL>SELECT * FROM table_a;
              ID             COL1             COL2
               1                2                0
               2                3                0
               3                1               15
SQL>SELECT * FROM table_b;
              ID             COLA
               1                6
               2               12
SQL>UPDATE table_a a
  2  SET col2 = nvl((SELECT a.col1*b.cola
  3              FROM table_b b
  4              WHERE a.id = b.id),a.col2);
3 rows updated.
SQL>commit;
Commit complete.
SQL>SELECT * FROM table_a;
              ID             COL1             COL2
               1                2               12
               2                3               36
               3                1               15In the case of table_b not having the id, the current value of col2 will be retained. If you want some other value, then substitute it in the NVL. You can even use a different select statement as the return from nvl.
SQL>UPDATE table_a a
  2  SET col2 = nvl((SELECT a.col1*b.cola
  3              FROM table_b b
  4              WHERE a.id = b.id),(SELECT col2 from table_a where id = 2));
3 rows updated.
SQL>commit;
Commit complete.
SQL>SELECT * FROM table_a;
              ID             COL1             COL2
               1                2               12
               2                3               36
               3                1               36