3

我有两张桌子：

[Table container]
id int
<some more fields>
latest_measurement int

[Table measurement]
id int
container_id int
unixtime bigint

现在我想latest_measurement根据测量表中的最新测量值更新传感器表列。我已经准备好子查询来返回最新的测量值sensor_id，这很有效：

SELECT m.fill_level
from measurement m
inner join (
    select m.container_id, MAX(m.unixtime) as maxdate from measurement m GROUP BY container_id
) m2
on m2.container_id = m.container_id
where m.unixtime = m2.maxdate

但是，如果我在更新语句中使用此查询，如下所示，它将失败并出现异常：

UPDATE container
SET latest_fill_level = (
    SELECT m.fill_level
    from measurement m
    inner join (
        select m.container_id, MAX(m.unixtime) as maxdate from measurement m GROUP BY container_id
) m2
    on m2.container_id = m.container_id
    where m.unixtime = m2.maxdate
    and container.id = m.container_id)

最后，这是一个例外：

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or when the subquery is used as an expression.

如何编写一个更新语句，该语句可以从产生多个值的 SELECT 语句中使用它们各自的值更新多行？

4

1 回答 1

2

您的子查询不会打破关系。如果有多个最新的测量值unixtime，它将返回多行。因为 SQL Server 不知道它应该使用哪一行来更新列，所以它会引发Subquery returned more than 1 value.错误。

你可以row_number()用来打破关系：

update  c
set     latest_measurement = m.fill_level
from    container c
join    (
        select  row_number() over (
                    partition by container_id
                    order by unixtime desc) as rn
        ,       *
        from    measurement
        ) m
on      c.id = m.container_id
        and m.rn = 1 -- Latest measurement only

或者，您可以使用top 1子句打破联系：

update  container
set     latest_measurement = 
        (
        select  top 1 fill_level
        from    measurement m
        where   container.id = m.container_id
        order by
                unixtime desc
        )
于 2013-06-06T07:21:58.673 回答