sql - 如何在没有游标的情况下获取一些记录，在 T-SQL 中没有交叉应用

Question

我有一个名为的表Objects，其中包含一些文件，例如：

1. 用户
2. 老师

还有另一个表 ( States) 保存这些对象的可能状态，例如：

1. 积极的
2. 闲
3. 教学
4. 休息
5. 创作

还有第三个表（联结表）记录每个对象的每个状态变化。在这第三个表 ( ObjectStates) 中，记录如下：

1. 1, 1, DateTime1（用户在 DateTime1 上处于活动状态）
2. 2, 5, DateTime2（教师在 DateTime2 上创作）

等等

现在，我想要的是获取每个对象及其最新状态（不是状态历史）的查询。cursors可以使用或 使用Cross Apply命令获得此结果。但是，我想知道是否有任何其他方法可以从这三个表中获取每个对象的最新状态？因为cursors贵。

Answer 1

使用row_number()窗口函数...

select * 
from
(

select objects.*,
       state.state,
       objectstates.changedate,
       row_number() over (partition by object.objectid order by changedate desc) rn
from 
    objects
         inner join
    objectstates
         on objects.id = objectstates.objectid
         inner join
    states 
         on objectstates.stateid = states.stateid
) v
where rn = 1

例如，如果row_number因为使用的是 SQL 2000 而无法使用，则可以对max/group by查询使用联接。

select objects.*,
       state.state,
       objectstates.changedate,
from 
    objects
         inner join
    objectstates
         on objects.id = objectstates.objectid
         inner join
    states 
         on objectstates.stateid = states.stateid
    inner join
         (select objectid, max(changedate) as maxdate from objectstates group by objectid) maxstates
         on objectstates.objectid=maxstates.objectid
         and objectstates.changedate = maxstates.maxdate

Answer 2

你可以在ObjectStates桌子上加入两次。表的第一个连接将获得max(activedate)for each objectid。第二次，您将同时加入 theobjectid和 的值，max(activedate)这将获得state与该值关联的值：

select o.name o_name,
  s.name s_name,
  os1.activedate
from objects o
left join
(
  select max(activeDate) activedate, objectid
  from objectstates
  group by objectid
) os1
  on o.id = os1.objectid
left join ObjectStates os2
  on os1.objectid = os2. objectid
  and os1.activedate = os2.activedate
left join states s
  on os2.stateid = s.id

请参阅带有演示的 SQL Fiddle

Answer 3

在过去的美好时光，我们只使用标量子查询。

select o.*, (select top(1) s.description
               from objectstates os
               join states s on s.id = os.state_id
              where os.object_id = o.id
           order by os.recorded_time desc) last_state 
  from objects o;

哪个 CROSS APPLY 取代。要将其扩展到更多领域，必须将其扩展为

select *
  from (
select o.*, (select top(1) os.id
               from objectstates os
              where os.object_id = o.id
           order by os.recorded_time desc) last_state 
  from objects o
       ) x
   join objectstates os on os.id = x.last_state
   join states s on s.id = os.state_id;

Answer 4

您可以使用 partition over 来查找每个对象的最新行，如下所示

create table #ObjectState
(
    Object int NOT NULL,
    State int NOT NULL,
    TimeStamp datetime NOT NULL
)

INSERT INTO #ObjectState (Object, State, TimeStamp) VALUES (1, 1, '2012-01-01')
INSERT INTO #ObjectState (Object, State, TimeStamp) VALUES (1, 2, '2012-01-02')
INSERT INTO #ObjectState (Object, State, TimeStamp) VALUES (1, 3, '2012-01-03')
INSERT INTO #ObjectState (Object, State, TimeStamp) VALUES (2, 4, '2012-01-01')
INSERT INTO #ObjectState (Object, State, TimeStamp) VALUES (2, 2, '2012-01-02')

select *, ROW_NUMBER() over (partition by Object order by TimeStamp desc) as RowNo from #ObjectState

select InnerSelect.Object, InnerSelect.State, InnerSelect.TimeStamp FROM
(
select *, ROW_NUMBER() over (partition by Object order by TimeStamp desc) as RowNo from #ObjectState
) InnerSelect
where InnerSelect.RowNo = 1


DROP TABLE #ObjectState

给出输出

Object  State  TimeStamp
1       3      2012-01-03 00:00:00.000
2       2      2012-01-02 00:00:00.000

最后选择