sql-server - SQL多次更新同一行

Question

一个例子，我们可以考虑对同一行进行多次更新的必要性:

create table t ( c1 int );
insert into t (c1) values (1);
insert into t (c1) values (2);
insert into t (c1) values (3);

update x1
set x1.c1 = X2.new
from t as x1
inner join
(
    select 1 as c1, 100 as new 
    union sel 1, 101
    union sel 1, 102
) as x2 on x1.c1 = x2.c1

同一行可能有 3 次更新。连接结果集有 3 次相同的行。这一行只访问过一次吗？最终值是如何计算的？

Answer 1

来自更新documentation :

Use caution when specifying the FROM clause to provide the criteria for the update operation. The results of an UPDATE statement are undefined if the statement includes a FROM clause that is not specified in such a way that only one value is available for each column occurrence that is updated, that is if the UPDATE statement is not deterministic.

在这种情况下，结果是不确定的。最好将这些 SQL 语句视为错误。