MERGE error : unable to get a stable set of rows in the source tables

Question

Hi,
For an update, the following MERGE statement throws the error-unable to get a stable set of rows in the source tables:
MERGE INTO table2t INT
USING (SELECT DISTINCT NULL bdl_inst_id,.......
FROM table1 ftp
WHERE ftp.gld_business_date = g_business_date
AND ftp.underlying_instrument_id IS NOT NULL) ui
ON ( ( INT.inst_id = ui.inst_id
AND g_business_date BETWEEN INT.valid_from_date
AND INT.valid_to_date
OR ( INT.ric = ui.ric
AND g_business_date BETWEEN INT.valid_from_date
AND INT.valid_to_date
OR ( INT.isin = ui.isin
AND g_business_date BETWEEN INT.valid_from_date
AND INT.valid_to_date
OR ( INT.sedol = ui.sedol
AND g_business_date BETWEEN INT.valid_from_date
AND INT.valid_to_date
OR ( INT.cusip = ui.cusip
AND g_business_date BETWEEN INT.valid_from_date
AND INT.valid_to_date
WHEN MATCHED THEN
UPDATE
SET INT.inst_id = ui.inst_id, INT.ric = ui.ric
WHEN NOT MATCHED THEN
INSERT (inst_key, ......)
VALUES (inst_key, ......);
To determine the existence of a record, first check if any match is found on the first key, if not then search for the second key and so on.
Now two records with differenct first key, i.e. inst_id, can have the same ric(second key). On a rerun, with the target table already populated, the code fails. The reason is it finds duplicate entries for the second key.
Any suggestions on how to make this work?
Thanks in advance.
Annie

Answer 1

Annie
You've spotted the problem (that two records have the same RIC). MERGE doesn't allow that; each record in the data being updated is only allowed to be updated once.
Is there a PK column (or columns) that we can rely on?
What you can try is to outer join FTP to INT. Something like:
MERGE INTO INT int1
USING (
    select columns_you_need
    from (
        select ftp.columns -- whatever they are
               , int2.columns
               , row_number() over (partition by int2.pk_columns order by int2.somecolumns) as rn
        from   ftp
        left join int int2
        on (the condition you used in your query)
    where rn=1
) s
WHEN MATCHED THEN UPDATE ...
WHEN NOT MATCHED THEN INSERT ...So if you can restrict the driving query so that only the first one of the possible updates actually gets presented to the MERGE operation, you might be in with a chance :-)
And of course this error is nothing to do with any triggers.
HTH
Regards Nigel