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select * from sys_mail_tb as a
left join sys_mail_readstate_tb
as b on a.mailid=b.mailid where
b.delstate is null
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select * from sys_mail_tb as a
left join sys_mail_readstate_tb
as b on a.mailid=b.mailid where
b.delstate <>1
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b.delstate <>1  ---------Ó¦¸Ãdelstate ΪnullµÄÊý¾ÝÒ²Òª²é³öÀ´°¡

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null ²ÎÓëµÄÔËËã½á¹û¶¼ÊÇ null  !
null ²»µÈÓÚÈκÎÆäËü¶ÔÏó£¬ÉõÖÁ null <> null
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