Sql 有效地选择最新答案
SQL Fiddle: 我有一张表格,里面有“你会参加这次活动吗?”这个问题的答案。每个用户可能会响应多次,所有答案都存储在表中。通常情况下,我们只对最新的答案感兴趣,我正试图为此构造一个高效的查询。我正在使用SQLServer2008R2 一个事件的表内容:Sql 有效地选择最新答案,sql,sql-server,sql-server-2008,tsql,Sql,Sql Server,Sql Server 2008,Tsql,SQL Fiddle: 我有一张表格,里面有“你会参加这次活动吗?”这个问题的答案。每个用户可能会响应多次,所有答案都存储在表中。通常情况下,我们只对最新的答案感兴趣,我正试图为此构造一个高效的查询。我正在使用SQLServer2008R2 一个事件的表内容: Column types: int, int, datetime, bit Primary key: (EventId, MemberId, Timestamp) 请注意,成员18首先回答“否”,然后回答“是”;成员20首先回答“是”
Column types: int, int, datetime, bit
Primary key: (EventId, MemberId, Timestamp)
请注意,成员18首先回答“否”,然后回答“是”;成员20首先回答“是”,然后回答“否”;成员11回答“否”,然后再次回答“否”。我想过滤掉这些成员的第一个答案。此外,可能有多个答案需要筛选-例如,用户可能会回答Yes、Yes、No、Yes、No、No
我尝试了一些不同的想法,并在SQLServerManagementStudio中对它们进行了评估,方法是输入所有查询,选择“显示估计的执行计划”,并比较每个查询的总成本(以百分比表示)。这是评估绩效的好方法吗
目前测试的不同查询:
-----------------------------------------------------------------
-- 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);
最好避免为每个成员使用子查询。在上一个查询中,我尝试使用groupby
,但仍然必须对答案列使用子查询。我真的想要这样的东西,但这不是有效的SQL,当然:
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);
选择a.EventId、a.MemberId、max(a.Timestamp)、a.AnswerSQLServer2008支持公共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
我也更喜欢CTE方法,但这里有另一个使用子查询的选项,应该可以工作:
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,我认为CTE的性能会更好
编辑--不再确定性能--下面是两者的示例--您可以看到各自的执行计划
祝你好运。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
)
演示为什么要存储所有答案的历史记录,而不仅仅是“当前”答案?主要是为了历史记录和可追踪性。很好,我现在已经将真实数据放到了SQL FIDLE上(问题编辑)。在SSMS中使用子查询运行myWhere、JW和您的查询将导致31%、38%和31%的成本,如果这是一个适当的性能度量。谢谢。根据SSMS,您的查询与SGEDES和my的成本相同,其中带有子查询。不客气。(根据您的数据),我的所有成本结果都不同;)那很有趣。我的表实际上包含大约1600行,但我认为一次选择一个事件(其中EventId=68)将使查询对所有查询都同样容易。那么,也许不是。