messageSendTime如果不为null，就把createDate换为messageSendTi

select distinct sendUserId,DATE_FORMAT(from_unixtime(createDate / 1000) ,'%Y-%m') as dateMonth,DATE_FORMAT(from_unixtime(createDate / 1000) ,'%Y-%m-%d') as dateDay from microschool.backend_notice whereDATE_FORMAT(from_unixtime(createDate / 1000) ,'%Y-%m-%d %H:%m:%s')>='2017-04-01 00:00:00'andDATE_FORMAT(from_unixtime(createDate / 1000),'%Y-%m-%d %H:%m:%s')<'2017-04-21 00:00:00'