使用select case語句進行SQL更新

Question

使用'select case'進行表更新時出現問題。

作爲一個正常的選擇case語句（不更新表）這個查詢工作完美：

select case 
    when t1.id in (select t2.id from t2 where [condition1] then 'aaa' 
    when t1.id in (select t3.id from t3 where [condition2] then 'bbb' 
    when t1.id in (select t4.id from t4 where [condition3] then 'ccc' 
    else 'ddd' 
end 
from owner.t1; 

然而，當我嘗試使用相同的「選擇情況」語句在更新語句，我得到一個錯誤，說明一個子查詢返回多於一行。這是不起作用的更新查詢：

update owner.t1 
set t1.var2 = 
(select case 
    when t1.id in (select t2.id from t2 where [condition1] then 'aaa' 
    when t1.id in (select t3.id from t3 where [condition2] then 'bbb' 
    when t1.id in (select t4.id from t4 where [condition3] then 'ccc' 
    else 'ddd' 
end 
from owner.t1); 

當我將代碼更改爲下面的代碼時，它的工作原理很慢，但速度非常慢。我的目的可能太慢了。

update owner.t1 
set t1.var2 = 
(select case 
    when t2.id in (select t2.id from t2 where [condition1] then 'aaa' 
    when t2.id in (select t3.id from t3 where [condition2] then 'bbb' 
    when t2.id in (select t4.id from t4 where [condition3] then 'ccc' 
    else 'ddd' 
end 
from owner.t2 
where t2.id = t1.id); 

所以我的問題是爲什麼我必須在輔助表中引用我的ID而不是我想要更新的表？這是額外的檢查在'where'語句中增加了額外的時間來操作？

Answer 1

爲什麼不只是這樣做呢？

update owner.t1 
    set t1.var2 = (case when t1.id in (select t2.id from t2 where [condition1] then 'aaa' 
         when t1.id in (select t3.id from t3 where [condition2] then 'bbb' 
         when t1.id in (select t4.id from t4 where [condition3] then 'ccc' 
         else 'ddd' 
        end);