Question

需要您的帮助才能知道是否可以从具有以下条件的表中选择值：

表格内容：2个对象之间的匹配（Id_obj_A; name_obj_A; country_obj_A; Id_obj_B; name_obj_B; country_obj_B）

Select *
from table
Where (only if country_obj_A <> country_obj_B)

非常感谢你的帮助

Answer 1

是。有几种方法，一种是使用NOT EXISTS这样：

select
       *
from tableA
where NOT EXISTS (
   select NULL
   from tableB
   where tableB.country_obj_B = tableA.country_obj_A
   )

或使用NOT IN

select
       *
from tableA
where country_obj_A NOT IN (
   select country_obj_B 
   from tableB
   )

或使用LEFT JOIN然后排除已加入的行：

select
       *
from tableA
left join tableB on tableA.country_obj_A = tableB.country_obj_B
where tableB.country_obj_B IS NULL