ऐसा कुछ करना शायद अधिक कुशल होगा
DELETE FROM transaction t1
WHERE EXISTS( SELECT 1
FROM transaction t2
WHERE t1.date = t2.date
AND t1.refnumber = t2.refnumber
AND t1.parentFolderId = t2.parentFolderId
AND t2.id > t1.id )
