嵌套選擇和引用到外部表

Question

我有以下的sql語句。我從一個平面樹結構中提取數據，我想選擇一個跟隨匹配abos_daten.erstellt = (select MAX(erstellt)...

的問題是，以選擇正確的MAX(erstellt)我需要以下條件where t2.parent_id = t1.parent_id。不幸的是t1不能被綁定，因爲它反對外部選擇語句。它似乎創造了一個圓圈。

select * from trees as t1 inner join abos_daten as starter on t1.parent_id = starter.abonr 
right outer join 
    (select * from trees as t3 inner join abos_daten on t3.child_id = abos_daten.abonr 
    where abos_daten.erstellt = (select MAX(erstellt) from abos_daten inner join trees as t2 on t2.child_id = abos_daten.abonr 
           where t2.parent_id = t1.parent_id and abos_daten.status_id <> 147 
           ) 
    ) as follower on t1.child_id = follower.abonr 

有沒有人知道如何解決這個問題？親切的問候， jonatan

Answer 1

首先，t1實際上並不指select語句;它是指from子句中的另一個實體。碰巧，SQL服務器有一個特別允許這種功能的語法：cross apply/outer apply。要在您的情況使用，你會想是這樣的（未經測試，因爲我不能重新創建表）：

select * 
from trees as t1 
    inner join abos_daten as starter 
     on t1.parent_id = starter.abonr 
    outer apply (select MAX(erstellt) as max_erstellt 
        from abos_daten 
         inner join trees as t2 
         on t2.child_id = abos_daten.abonr 
        where t2.parent_id = t1.parent_id 
         and abos_daten.status_id <> 14) as t_m 
    right outer join (select * 
         from trees as t3 
          inner join abos_daten 
           on t3.child_id = abos_daten.abonr) as follower 
     on t1.child_id = follower.abonr 
      and t_m.max_erstellt = follower.erstellt