结论：关联键 NULL = NULL时，不进行关联，即两表关联失败

案例如下：

表A

表B

 

表A 关联 表B

select
       a.id      as  a_id
      ,a.name    as  a_name
      ,b.id      as  b_id
      ,b.name    as  b_name
from
    表A  a
left join
    表B  b
on  a.id = b.id
and a.name = b.name

 结果

 结论：关联键 NULL = NULL时，不进行关联，即两表关联失败