[英]Select Most Recent States From History Table
我繼承了一個結構如下的表:
ID Name Timestamp Data
----------------------------
1 A 40 ...
2 A 30 ...
3 A 20 ...
4 B 40 ...
5 B 20 ...
6 C 30 ...
7 C 20 ...
8 C 10 ...
ID
是標識字段和主鍵, Name
和Timestamp
字段上有非唯一索引。
什么是最有效的方式來獲取最新的每個項目名稱記錄,即在上述行1,因為它們分別為項目A,B和C的最先進的最新條目4和6應該返回的表。
SQL Server 2005(以后):
WITH MostRecentRows AS
(
SELECT ID, Name, Data,
ROW_NUMBER() OVER (PARTITION BY Name ORDER BY TimeStamp DESC) AS 'RowNumber'
FROM MySchema.MyTable
)
SELECT * FROM MostRecentRows
WHERE RowNumber = 1
假設每個名稱沒有重復的時間戳,這樣的事情應該有效:
SELECT ID, Name, Timestamp, Data
FROM test AS o
WHERE o.Timestamp = (SELECT MAX(Timestamp)
FROM test as i
WHERE i.name = o.name)
SQL Server 2000:
SELECT
ID, Name, Timestamp, Data
FROM
DataTable
INNER JOIN
(
SELECT ID, MAX(Timestamp) Timestamp FROM DataTable GROUP BY ID
) latest ON
DataTable.ID = Latest.ID AND
DataTable.Timestamp = Latest.Timestamp
另一個簡單方法:
SELECT ID,Name,Timestamp, Data
FROM Test_Most_Recent
WHERE Timestamp = (SELECT MAX(Timestamp)
FROM Test_Most_Recent
group by Name);
如果您使用的是SQL Server 2005/2008,那么從性能角度來看,Mitch Weat已經列出的CTE解決方案是最好的。 但是,如果您使用的是SQL Server 2000,那么您不能假設沒有重復的Name | TimeStamp組合。 使用以下代碼每個名稱只返回一條記錄:
SELECT ID
, Name
, TimeStamp
, Data
FROM DataTable dt
INNER JOIN
(SELECT Name
, MIN(DataTable.ID) AS MinimumID
FROM DataTable
INNER JOIN
(SELECT Name
, MAX(Timestamp) AS Timestamp
FROM DataTable
GROUP BY Name) latest
ON DataTable.Name = Latest.Name
AND DataTable.Timestamp = Latest.Timestamp
GROUP BY Name) MinimumLatest
ON dt.ID = MinimumLatest.ID
因此,如果您添加另一條記錄,例如9 C 30,那么這只會返回ID 6.如果您沒有這么做,那么您最終可能會返回9 C 30和6 C 30。
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