Mysql如何查询数据关联？

select * from 表名 where toId in(select fromId from 表名 where toId = 3) or toId = 3

或

select * from 表名 where toId in((select fromId from 表名 where toId = 3), 3)

内关联 inner join

左关联 left join

右关联 right join

交叉关联 cross join

语法为：

select *

from a inner join b

on a.xx=b.xx

where XXX

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