SELECT TO_CHAR(dateTime, 'DD') DAY, TO_CHAR(dateTime, 'MM') MONTH, errormessage
FROM log
WHERE (...)
GROUP BY TO_CHAR(dateTime, 'DD'), TO_CHAR(dateTime, 'MM'), errormessage

列别名不适用于GROUP BY，你需要完整的表达式.

Column aliases are no good for GROUP BY, you need the full expression.