问题描述
|
select first(orderid),accountid
from [Order]
group by AccountId
order by DateCreated desc
first()是无效函数。
max()不适用于唯一标识符
如何获得为所有帐户创建的最后一个订单编号?谢谢。
解决方法
像(未经测试的):
;WITH CTE_LatestOrders AS (
select accountid,lastcreated = max(datecreated)
from [Order]
group by accountid
)
select
accountid,orderid
from
[Orders] o
join CTE_LatestOrders l
on o.AccountID = l.AccountID
and o.datecreated = l.lastcreated
,您也可以继续下面的操作。
Select Temp.orderid,T.AccountId,T.DateCreated
From
(
Select AccountId,max(DateCreated) as DateCreated
From [Order]
Group By AccountId
)T
Inner Join [Order] Temp on Temp.AccountId = T.AccountId
AND Temp.DateCreated = T.DateCreated
CTE不是UDT /临时表;将CTE视为仅为当前查询定义的视图。就像视图一样,将CTE展开并折叠到整个查询计划中。全局优化仍然会发生,但是不要认为仅仅因为您使用了CTE,您就只会执行一次查询。这是适合该空间的一个简单示例:WITH vw AS(SELECT COUNT(*)c FROM Person)SELECT a.c,b.c FROM vw a,vw b;查询计划将清楚地显示两个扫描/聚合和一个联接,而不是仅将相同的结果投影两次。
,从MS SQL 2012开始,Max()确实适用于唯一标识符