Update Statement: Multi Level Inline view.

Question

Here is the test case.
create table source(old_val integer, new_val integer)
create table target(val integer)
insert into target values(1)
insert into target values(2)
insert into target values(3)
insert into source values(1,4)
insert into source values(1,5)
insert into source values(2,6)
insert into source values(3,7)
insert into source values(3,8)
commit
/ Now i want to update target.val with source.val and the matching condition is source.old_val = target.val
But source.old_val may have multiple values for a given target.val. You can see that in the example. So i want to order by ROWID of source and pick the first record.
So i wrote a UPDATE.
SQL> update target t
  2     set (val) = (select new_val
  3               from (select new_val, row_number() over(order by rowid) rno
  4                       from source s
  5                      where s.old_val = t.val)
  6              where rno = 1)
  7   where exists(select null
  8             from source s
  9            where t.val = s.old_val)
10  /
3 rows updated.and this is what i got.
SQL> select * from target
  2  /
       VAL
         4
         4
         4
SQL> select * from source
  2  /
   OLD_VAL    NEW_VAL
         1          4
         1          5
         2          6
         3          7
         3          8i was expecting the value to be updated as 4,6,7 or 5,6,8 but not sure why its 4,4,4.
Just trying to understand how this update is being processed. Not sure if i am missing something basic.
NOTE: I am interested in knowing the behavior of this update statement and not an alternative method to solve the issue.
Thanks,
Karthick.

Answer 1

>
Karthick_Arp wrote
i was expecting the value to be updated as 4,6,7 or 5,6,8 but not sure why its 4,4,4.
>
if you want 4,5,6 , why you are defining RNO = 1!?
SQL> SELECT new_val
  2    FROM (SELECT new_val
  3                ,row_number() over(Order BY s.Rowid) rno
  4            FROM Source s
  5                ,target t
  6           WHERE s.old_val = t.val)
  7   WHERE rno = 1
  8  /
                                NEW_VAL
                                      4look at this,
SQL> SELECT new_val
  2    FROM (SELECT new_val
  3                ,row_number() over(Order BY s.Rowid) rno
  4            FROM Source s
  5                ,target t
  6           WHERE s.old_val = t.val)
  7   WHERE rno <= 3
  8  /
                                NEW_VAL
                                      4
                                      5
                                      6Regards,
Christian Balz
Edited by: Christian Balz on 22/06/2009 06:06