Question

几个小时我一直在研究这个问题。

我在MySQL中有一个名为person_state的表

----------
person_state
----------------------------
id | person_id | state | date
----------------------------

我需要活跃的人员上次激活日期，每个人的状态已被多次更改（状态为：主动，被动，等待，阻止）。如果一个人被激活然后被停用，我的查询就不应该得到它。

我的查询是

select id as activation_id, person_id as active_person_id
  from person_state
  where state = 'active' 
  and
  not exists(
         select * from person_state 
         where person_id = active_person_id
         and
         id > activation_id
    )

我在'where子句' 中收到错误 未知列'active_person_id'。

感谢您的时间

Answer 1

WHERE在SELECT之前执行，因此，您不能在其中使用列别名，请尝试使用列名，例如：

select id as activation_id, person_id as active_person_id
  from person_state ps
  where state = 'active' 
  and
  not exists(
         select id from person_state 
         where person_id = ps.person_id
         and
         id > ps.activation_id
    )

Answer 2

您可以使用以下查询

select P.id as activation_id, P.person_id as active_person_id
  from person_state P
  where state = 'active' 
  and
  not exists(
         select 1 from person_state 
         where person_id = P.active_person_id
         and
         id > P.activation_id
    );

您错过了提供别名。您可以给1或*来选择日期。但是1检查存在时运行速度更快，并且只搜索一列

Answer 3

如果表中有多个列，则应使用限定列名：

select ps.id as activation_id, ps.person_id as active_person_id
from person_state ps
where ps.state = 'active' and
      not exists (select *
                  from person_state ps2
                  where ps2.person_id = ps.person_id and
                        ps2.id > ps.id
                 ) ;

您的主要问题是您无法在子查询中使用列别名。

MySQL不存在未知列错误

3 个答案: