Question

我有以下SQL

SELECT
    Seq.UserSessionSequenceID, 
    Usr.SessionGuid, 
    Usr.UserSessionID,
    Usr.SiteID, 
    Seq.Timestamp, 
    Seq.UrlTitle, 
    Seq.Url
FROM
    tblUserSession Usr
INNER JOIN  
    tblUserSessionSequence Seq ON Usr.UserSessionID = Seq.UserSessionID
WHERE     
    (Usr.Timestamp > DATEADD(mi, -45, GETDATE())) AND (Usr.SiteID = 15)
ORDER BY Usr.Timestamp DESC

非常简单的东西。在tblUserSessionSequence中本质上有多个UserSessionIDs行。我只想返回具有唯一UserSessionID的最新（前1）行。我该怎么做？

Answer 1

您可以使用窗口函数ROW_NUMBER为每个用户编号行，并仅选择行号为1的行。

SELECT
    UserSessionSequenceID, 
    SessionGuid, 
    UserSessionID,
    SiteID, 
    Timestamp, 
    UrlTitle, 
    Url
FROM (
    SELECT
        Seq.UserSessionSequenceID, 
        Usr.SessionGuid, 
        Usr.UserSessionID,
        Usr.SiteID, 
        Usr.Timestamp AS UsrTimestamp, 
        Seq.Timestamp, 
        Seq.UrlTitle, 
        Seq.Url,
        ROW_NUMBER() OVER (PARTITION BY Usr.UserSessionID
                           ORDER BY Seq.UserSessionSequenceID DESC) AS rn
    FROM
        tblUserSession Usr
    INNER JOIN  
        tblUserSessionSequence Seq ON Usr.UserSessionID = Seq.UserSessionID
    WHERE     
        (Usr.Timestamp > DATEADD(mi, -45, GETDATE())) AND (Usr.SiteID = 15)
) T1
WHERE rn = 1
ORDER BY UsrTimestamp DESC

Answer 2

如果您希望在查询中只返回一行（即带有最新时间戳的ID），只需更改

SELECT

到

SELECT TOP 1

如果您希望为每个UserSessionID 获取单行，但是您希望确保获得具有最新TimeStamp的那一行，则会稍微复杂一些。

你可以这样做：

SELECT Seq.UserSessionSequenceID, Usr.SessionGuid, Usr.UserSessionID, Usr.SiteID, Seq.Timestamp, Seq.UrlTitle, Seq.Url FROM tblUserSession Usr INNER JOIN (SELECT UserSessionSequenceID, UserSessionID, Timestamp, UrlTitle, Url, ROW_NUMBER() over (PARTITION BY UserSessionID ORDER BY UserSessionSequenceID) AS nbr FROM tblUserSessionSequence) Seq ON Usr.UserSessionID = Seq.UserSessionID AND Seq.nbr = 0 WHERE (Usr.Timestamp > DATEADD(mi, -45, GETDATE())) AND (Usr.SiteID = 15) ORDER BY Usr.Timestamp DESC

SQL子查询问题

2 个答案: