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
-----------------------------------------可以查到select * from sys_mail_tb as a
left join sys_mail_readstate_tb
as b on a.mailid=b.mailid where
b.delstate<>1
-----------------------------------------查到不到b.delstate<>1 ---------应该delstate 为null的数据也要查出来啊
left join sys_mail_readstate_tb
as b on a.mailid=b.mailid where
b.delstate is null
-----------------------------------------可以查到select * from sys_mail_tb as a
left join sys_mail_readstate_tb
as b on a.mailid=b.mailid where
b.delstate<>1
-----------------------------------------查到不到b.delstate<>1 ---------应该delstate 为null的数据也要查出来啊
解决方案 »
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null 不等于任何其它对象,甚至 null <> null