left semi join 的结果只返回左表的列，右表只能放在 join condition 中用于过滤判断，可以理解为 in 的替代品

select  *
from a left semi join b
on a.id = b.id

--- 等价于

select * 
from a 
where a.id in 
(select b.id from b)

left anti join 与 left semi join 相反，返回不满足 join condition 的结果，可以理解为 not in 的替代品

select  *
from a left anti join b
on a.id = b.id

--- 等价于

select * 
from a 
where a.id not in
(select b.id from b)