Oracle 11g Release 2

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ALTER SESSION SET NLS_DATE_FORMAT = 'yyyymmdd';
SELECT *
FROM   DUAL
WHERE  MAX(dummy) OVER(PARTITION BY dummy) >= 'X'
ORA-00934: group function is not allowed here

SELECT dummy
FROM   (
  SELECT dummy,
         MAX(dummy) OVER(PARTITION BY dummy) AS max_dummy 
  FROM   DUAL
)
WHERE  max_dummy >= 'X'


DUMMY


X