使用select case语句进行SQL更新

Question

使用'select case'进行表更新时出现问题。

作为一个正常的选择case语句（不更新表）这个查询工作完美：

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，当我尝试使用相同的“选择情况”语句在更新语句，我得到一个错误，说明一个子查询返回多于一行。这是不起作用的更新查询：

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); 

当我将代码更改为下面的代码时，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); 

所以我的问题是为什么我必须在辅助表中引用我的ID而不是我想要更新的表？这是额外的检查在'where'语句中增加了额外的时间来操作？

Answer 1

为什么不只是这样做呢？

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);