Merge query returning ORA-30926: unable to get a stable set of rows in the source tables

Question

I have two merge queries which triggers one after another

First Query

merge into MyTable
using
(   
select distinct nullLogSetId.Id         


        

Answer 1

if you see i used distinct in both of the queries so duplicating of rows is not an issue.

You probably have duplicates in data. DISTINCT does not guarantee you have IdToUpdate unique when you use it with other columns. See:

CREATE TABLE #MyTable(IdToUpdate INT, LogSetIdToUpdateTo INT);

INSERT INTO #MyTable VALUES (1,1), (1,2), (2,1),(3,1);

SELECT DISTINCT IdToUpdate, LogSetIdToUpdateTo
FROM #MyTable;

LiveDemo

You will get IdToUpdate twice. Check your data:

with cte AS (
  select distinct nullLogSetId.Id as IdToUpdate,
                  knownLogSetId.LogSetId LogSetIdToUpdateTo
  from MyTable knownLogSetId 
  join MyTable nullLogSetId
    on knownLogSetId.IdentifierType = nullLogSetId.IdentifierType 
   and knownLogSetId.Identifier = nullLogSetId.Identifier 
  where 
    knownLogSetId.IdentifierType = 'DEF'
    and knownLogSetId.LogSetId >= 0 
    and nullLogSetId.LogSetId = -1
)
SELECT IdToUpdate, COUNT(*) AS c
FROM cte
GROUP BY IdToUpdate
HAVING COUNT(*) > 1;

One way to go is to use aggregation function(MAX/MIN) instead of DISTINCT:

merge into MyTable
using
(

  select nullLogSetId.Id as IdToUpdate,
         MAX(knownLogSetId.LogSetId) AS LogSetIdToUpdateTo 
  from MyTable knownLogSetId 
  join MyTable nullLogSetId
    on knownLogSetId.IdentifierType = nullLogSetId.IdentifierType 
   and knownLogSetId.Identifier = nullLogSetId.Identifier 
  where 
    knownLogSetId.IdentifierType = 'DEF'
    and knownLogSetId.LogSetId >= 0 
    and nullLogSetId.LogSetId = -1
  GROUP BY nullLogSetId.Id
) on (Id = IdToUpdate)
when matched then
update set LogSetId = LogSetIdToUpdateTo