Question

我有一个表格，其中包含“你会参加这个活动吗？”这个问题的答案。每个用户可能会多次响应，所有答案都会存储在表格中。通常我们只对最新的答案感兴趣，并且我正在尝试构建一个有效的查询。我正在使用SQL Server 2008 R2。

一个事件的表格内容：

Table contents

Column types: int, int, datetime, bit
Primary key: (EventId, MemberId, Timestamp)

请注意，会员18首先回答否及以后是，会员20首先回答是，然后回答否，会员11回答否，之后回答否。我想过滤掉这些成员的第一个答案。此外，可能有多个应该过滤的答案 - 例如，用户可能会回答是，是，否，是，否，否，否。

我尝试了一些不同的想法，并通过输入所有查询，选择显示估计执行计划并比较每个查询的总成本（百分比），在SQL Server Management Studio中对它们进行了评估。这是评估绩效的好方法吗？

到目前为止测试的不同查询：

-----------------------------------------------------------------
-- Subquery to select Answer (does not include Timestamp)
-- Cost: 63 %
-----------------------------------------------------------------
select distinct a.EventId, a.MemberId,
(
  select top 1 Answer
  from    Attendees
  where EventId   = a.EventId
  and   MemberId  = a.MemberId
  order by Timestamp desc
) as Answer
from    Attendees a
where a.EventId = 68

-----------------------------------------------------------------
-- Where with subquery to find max(Timestamp)
-- Cost: 13 %
-----------------------------------------------------------------
select a.EventId, a.MemberId, a.Timestamp, a.Answer
from     Attendees a
where  a.EventId = 68
and    a.Timestamp =
(
  select max(Timestamp)
  from     Attendees
  where  EventId  = a.EventId
  and    MemberId = a.MemberId
)
order by a.TimeStamp;

-----------------------------------------------------------------
-- Group by to find max(Timestamp)
-- Subquery to select Answer matching max(Timestamp)
-- Cost: 23 %
-----------------------------------------------------------------
select a.EventId, a.MemberId, max(a.Timestamp),
(
  select top 1 Answer
  from    Attendees
  where EventId   = a.EventId
  and   MemberId  = a.MemberId
  and   Timestamp = max(a.Timestamp)
) as Answer
from    Attendees a
where a.EventId = 68
group by a.EventId, a.MemberId
order by max(a.TimeStamp);

避免为每个成员使用子查询会很好。在上一个查询中，我尝试使用group by，但仍然必须使用“回答”列的子查询。我真的很喜欢这样的东西，但那当然不是有效的SQL：

select a.EventId, a.MemberId, max(a.Timestamp), a.Answer <-- Picked from the line selected by max(a.Timestamp)
from  Attendees a
where a.EventId = 68
group by a.EventId, a.MemberId
order by max(a.TimeStamp);

有效查询的其他任何想法？

编辑：

SQL Fiddle印象非常深刻，我现在输入了我的实际数据： http://sqlfiddle.com/#!3/9b459/6

Answer 1

SQL Server 2008支持公用表表达式和窗口函数。

WITH recordsList
AS
(
    SELECT  EventID, MemberID, TimeStamp, Answer,
            ROW_NUMBER() OVER (PARTITION BY EventID, MemberID
                                ORDER BY Timestamp DESC) rn
    FROM    tableName
)
SELECT  EventID, MemberID, TimeStamp, Answer
FROM    recordsList
WHERE   rn = 1

Answer 2

我也更喜欢CTE方法，但这是使用应该有效的子查询的另一种选择：

SELECT T.EventId, T.MemberId, T.TimeStamp, T.Answer
FROM TableName T
 JOIN (
   SELECT EventId, MemberId, Max(Timestamp) MaxTimeStamp
   FROM TableName
   GROUP BY EventId, MemberId ) T2 ON T.EventId = T2.EventId 
    AND T.MemberId = T2.MemberId 
    AND T.TimeStamp = T2.MaxTimeStamp

话虽如此，我想CTE会有更好的表现。

编辑 - 不再确定性能 - 这两者都是SQL Fiddle - 您可以看到每个的执行计划。

祝你好运。

Answer 3

还有一个选择

SELECT a.EventId, a.MemberId, a.Timestamp, a.Answer
FROM Attendees a
WHERE a.EventId = 68 AND EXISTS (
              SELECT 1
              FROM Attendees
              WHERE EventId = a.EventId             
              GROUP BY MemberId
              HAVING  MAX(Timestamp) = a.Timestamp                      
                      AND MemberId  = a.MemberId
              )

SQLFiddle上的演示

有效地选择最近的答案

3 个答案: