Question

使用“选择案例”进行表格更新时遇到问题。

作为正常的选择案例语句（不更新表），此查询可以完美运行：

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“select case”语句时，我收到一条错误，指出子查询返回的行数超过1行。这是无效的更新查询：

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时，它的工作速度非常慢。可能太慢了我的目的。

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？这是在'where'语句中额外检查，为操作增加了很多额外的时间吗？

Answer 1

为什么不这样做？

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);

使用select case语句进行SQL更新

1 个答案: