使用 select case 语句进行 SQL 更新

Question

使用“选择案例”进行表更新时遇到问题。

作为一个普通的 select case 语句（不更新表），这个查询完美地工作：

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;

但是，当我尝试在更新语句中使用相同的“选择案例”语句时，我收到一条错误消息，指出子查询返回多于 1 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);

当我将代码更改为以下代码时，它的运行速度非常慢。可能对我的目的来说太慢了。

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 而不是我要更新的表？ 'where' 语句中的这种额外检查是否会为操作增加大量额外时间？

Answer 1

为什么不这样做呢？

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);