左连接条件的执行顺序？ 测试中，300条数据成功，700条数据出错(substring不能为空)

select *
from tb1
left join tb2
on tb1.str != null
and substring(str, 0, 2) = tb2.str.

on tb1.str is not null
and tb2.str is not null
and substring(tb1.str, 0, 2) = tb2.str.