僅在滿足特定條件時選擇最后記錄

Question

我有一個包含ID，WorkerID，IsActive標志（1或0）和LocalTime的表。 每次工作人員處於活動狀態或不處於活動狀態時，都會使用WorkerID，1或0標志記錄以及時間（LocalTime）創建一條記錄。

我想插入一個新表中：從該表中，對於每個唯一的WorkerID，僅當該記錄的IsActive標志為1時，才為該唯一的WorkerID選擇具有最新LocalTime的記錄的WorkerID，LocalTime和LocalTime。對於WorkerID，具有最新LocalTime的記錄的IsActive值為1，請選擇它。 否則，如果對於WorkerID，具有最新LocalTime的記錄的IsActive值為0，請不要選擇它。

例：

ID  WorkerID    IsActive    LocalTime
1   11      1       21/04/2015
2   22      0       22/04/2015
3   11      0       21/04/2015
4   22      1       22/04/2015

只能選擇ID為4的記錄。

Answer 1

--get unique worker ids
select distinct WorkerId
from MyTable a
--which are active
where IsActive=1
--that don't have any worker ids on a more recent time
--(ignoring status as if there's a more recent active record for 
--this worker we want that record, not this one; if there's a more 
--recent inactive record we don't want this one anyway)
and not exists
(
    select top 1 1
    from MyTable b
    where b.WorkerId = a.WorkerId
    and b.LocalTime > a.LocalTime
)

Answer 2

使用CROSS APPLY ：

Select * From Worker w1
Cross Apply(Select * From(Select Top 1 * From Worker w2 
                          Where w1.WorkerID = w2.WorkerID
                          Order By w2.ID desc) t 
            Where t.IsActive = 1 And t.ID = w1.ID) ca