我认为您的问题应该是

select * 
from (
    select a.* 
    from sessions as a
    join
        (select b.customer_id, min(b.timestamp), b.marketing_source as first_touch_source, b.marketing_medium as first_touch_medium 
        from sessions as b
        group by b.customer_id
    ) c USING (customer_id) # or c.customer_id = a.customer_id
) as T

• 如果你只是想把最小日期列，你可以尝试两种方法，第二种方法将工作，如果你的版本支持windows功能
• 子查询总是很混乱，所以我建议使用cte（如果支持的话

第一个

一些DBMS要求您命名所有派生表。您的查询（我删除了不必要的派生表T）：

select * 
from (select a.* from sessions as a)
join (select b.customer_id, min(b.timestamp)
           , b.marketing_source as first_touch_source
           , b.marketing_medium as first_touch_medium 
      from sessions as b
      group by b.customer_id
     )
  on a.customer_id = b=customer_id

可更改为：

select * 
from (select a.* from sessions as a) AS c
join (select b.customer_id, min(b.timestamp)
           , b.marketing_source as first_touch_source
           , b.marketing_medium as first_touch_medium 
      from sessions as b
      group by b.customer_id
     ) AS d
  on c.customer_id = d.customer_id

为避免混淆，应在外部级别选择另一个别名，尽管内部别名在那里不可见。
附注：衍生表格d可能是也可能不是有效的SQL。SQL92中不允许使用衍生表格d，但如果marketing_* 在功能上相依于customer_id，则SQL99中允许使用衍生表格d。
您可以进一步将其简化为：

select * 
from sessions AS c
join (select b.customer_id, min(b.timestamp) as ts
           , b.marketing_source as first_touch_source
           , b.marketing_medium as first_touch_medium 
      from sessions as b
      group by b.customer_id
     ) AS d
  on c.customer_id = d.customer_id

我假设您还想使用c.timestamp = d.ts进行连接。如果是这种情况，并且您使用的是最新版本的MySQL（8+），则可以使用窗口函数而不是自连接

select customer_id, ...
from (
    select b.customer_id
         , b.marketing_source
         , b.marketing_medium
         , row_number() over (partition by customer_id
                              order by b.timestamp) as rn 
    from sessions as b
) as T
where rn = 1