ORA-38104 when trying to update my table using merge - sql

Question

I have a stored procedure in which I want to update some columns, so I wrote below code:
PROCEDURE UPDATE_MST_INFO_BKC (
P_SAPID IN NVARCHAR2
) AS
BEGIN
MERGE INTO tbl_ipcolo_billing_mst I
USING (
SELECT
R4G_STATE, -- poilitical state name
R4G_STATECODE, -- poilitical state code
CIRCLE, -- city name
NE_ID,
LATITUDE,
LONGITUDE,
SAP_ID
FROM
R4G_OSP.ENODEB
WHERE
SAP_ID = P_SAPID
AND ROWNUM = 1
)
O ON ( I.SAP_ID = O.SAP_ID )
WHEN MATCHED THEN
UPDATE SET I.POLITICAL_STATE_NAME = O.R4G_STATE,
I.POLITICAL_STATE_CODE = O.R4G_STATECODE,
I.CITY_NAME = O.CIRCLE,
I.NEID = O.NE_ID,
I.FACILITY_LATITUDE = O.LATITUDE,
I.FACILITY_LONGITUDE = O.LONGITUDE,
I.SAP_ID = O.SAP_ID;
END UPDATE_MST_INFO_BKC;
But it is giving me error as
ORA-38104: Columns referenced in the ON Clause cannot be updated: "I"."SAP_ID"
What am I doing wrong?

Answer 1

You are joining the source to destination tables on I.SAP_ID = O.SAP_ID and then, when matched, are trying to update them and set I.SAP_ID = O.SAP_ID. You cannot update the columns used in the join ... and why would you want to as you have already determined that the values are equal.
Just remove the last line of the UPDATE:
...
O ON ( I.SAP_ID = O.SAP_ID )
WHEN MATCHED THEN
UPDATE SET I.POLITICAL_STATE_NAME = O.R4G_STATE,
I.POLITICAL_STATE_CODE = O.R4G_STATECODE,
I.CITY_NAME = O.CIRCLE,
I.NEID = O.NE_ID,
I.FACILITY_LATITUDE = O.LATITUDE,
I.FACILITY_LONGITUDE = O.LONGITUDE;

Answer 2

The error message tells you what the problem is - a MERGE statement cannot update the columns used in the ON clause - and even tells you what column is the problem: "I"."SAP_ID".
So Oracle hurls ORA-38104 because of this line in your WHEN MATCHED branch
I.SAP_ID = O.SAP_ID;
Remove it and your problem disappears. Fortunately the line is unnecessary: I.SAP_ID already equals O.SAP_ID, otherwise the record wouldn't go down the MATCHED branch.
The reason why is quite straightforward: transactional consistency. The MERGE statement operates over a set of records defined by the USING clause and the ON clause. Updating the columns used in the ON clause threatens the integrity of that set, and so Oracle forbids it.