Problem in merge condition in oracle when merging two tables - oracle

Question

I have a table which has data as:
id payor_name
---------------
1 AETNA
2 UMR
3 CIGNA
4 METLIFE
4 AETNAU
5 ktm
6 ktm
Id and payor_name are two columns.So,
My expected output is:
id payor_name
---------------
1 AETNA
2 UMR
3 CIGNA
4 METLIFE
4 AETNAU
6 ktm ...> I want to change the id of this row to be 6 from 5.
6 ktm
I want one to one mapping between id and payor_name.So,this is what I tried:
MERGE INTO offc.payor_collec A
USING (select id from offc.payor_collec where payor_name in(
select payor_name from offc.payor_collec group by payor_name having count(distinct id)>=2)) B
ON (A.id=B.id)
WHEN MATCHED THEN
UPDATE SET A.id=B.id
But when I compiled I got error as:
Error at line 1
ORA-38104: Columns referenced in the ON Clause cannot be updated: "A"."ID"
Id is number where as payor_name is varchar2.
How can I achieve this result?

Answer 1

MERGE works, but slightly different than your code.
SQL> select * from test;
ID PAYOR
---------- -----
1 aetna
2 umr
5 ktm
6 ktm
SQL> merge into test t
2 using (select max(t1.id) id,
3 t1.payor_name
4 from test t1
5 group by t1.payor_name
6 ) x
7 on (x.payor_name = t.payor_name)
8 when matched then update set
9 t.id = x.id;
4 rows merged.
SQL> select * from test;
ID PAYOR
---------- -----
1 aetna
2 umr
6 ktm
6 ktm
SQL>

Answer 2

Use a correlated subquery:
UPDATE PAYOR_COLLEC pc
SET pc.ID = (SELECT MAX(pc2.ID)
FROM PAYOR_COLLEC pc2
WHERE pc2.PAYOR_NAME = pc.PAYOR_NAME)
dbfiddle here

Answer 3

You can use a MERGE statement, as you tried and as Littlefoot has shown.
You can also use a correlated subquery as Bob Jarvis has shown, but that will be quite inefficient.
Many Oracle developers are unaware that you can also update through a join. Worse, there are many who say "there is no such thing in Oracle."
In your problem, you need to join your table to an aggregate query (picking just the max id for each payor_name) and the join is on the group by column in the aggregate. This already guarantees that the join column will be unique in the right-hand table; that is all Oracle needs to allow the update through join.
Here is a complete example, starting with the create table statement, then the update and then showing the table after the update. Note that I don't need any constraints (like primary key, not null, unique, etc.) or indexes on the base table. If they do exist, so much the better, but the solution works in the most general case.
create table t (id, payor_name) as
select 1, 'AETNA' from dual union all
select 2, 'UMR' from dual union all
select 3, 'CIGNA' from dual union all
select 4, 'METLIFE' from dual union all
select 4, 'AETNAU' from dual union all
select 5, 'ktm' from dual union all
select 6, 'ktm' from dual;
Table T created.
update
(
select id, payor_name, max_id
from t inner join
(select max(id) as max_id, payor_name from t group by payor_name)
using (payor_name)
)
set id = max_id where id != max_id
;
1 row updated.
select * from t;
ID PAYOR_NAME
----- ----------
1 AETNA
2 UMR
3 CIGNA
4 METLIFE
4 AETNAU
6 ktm
6 ktm
Notice the where clause at the end of the update statement, too. You don't want to update rows to their pre-existing value; that will still generate undo and redo data (although I understand that Oracle has changed that in more recent versions - it now doesn't generate undo and redo unless a row did indeed change). I assume ID is NOT NULL - otherwise you should rewrite the where clause as
where decode(id, max_id, 0) is null
or equivalent