mysql中datetime比較大小問題

select sum(studychj) as tofflinejz from afterline where checkflag=1 and studytype=1 and studybegin>='2010-01-01 00:00:00' and studyend<='2010-12-01 00:00:00' and username='01009' 這麼寫不報錯，但得出結果不許確函數    

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