如何使用生成多行的select語句更新表？

Question

我有兩個表：

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

[Table measurement] 
id int 
container_id int 
unixtime bigint 

現在我想根據從測量表測量的最新更新傳感器表latest_measurement列。我已經準備了子查詢每sensor_id返回最新的測量和這個作品：

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，如果我再在我的更新語句中使用此查詢，如下所示時，出現異常：

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) 

最後，這裏的例外：

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

如何編寫可以從SELECT語句塔更新各自的價值多行的更新語句t產生多個值？

Answer 1

您的子查詢不會斷開關係。如果最近的unixtime有多個測量結果，它將返回多行。由於SQL Server無法知道應該使用哪一行來更新列，因此會引發Subquery returned more than 1 value.錯誤。

你可以使用row_number()打破平局：

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，你可以打破與top 1條款的關係：

update container 
set  latest_measurement = 
     (
     select top 1 fill_level 
     from measurement m 
     where container.id = m.container_id 
     order by 
       unixtime desc 
     )