SQL子查询问题

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。tblUserSessionSequence中本质上有多个UserSessionID行。我只想返回具有唯一UserSessionID的最新（前1）行。如何操作？

您可以使用窗口功能为每个用户的行编号，并仅选择行编号为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

---

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

如果您希望为每个UserSessionID获取一行，但希望确保获取具有最新

时间戳的一行，那么这就稍微复杂一些

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 

最新的UserSessionID可能与最高的UserSessionSequenceId重复请允许我再指定一点。我仍然希望在总体结果中有多行。只需TblUserSessionSequenced中的第一个唯一UserSessionID，当我尝试使用您提供的上述代码运行它时，就会出现此错误：Msg 1033，级别15，状态1，第21行ORDER BY子句在视图、内联函数、派生表、子查询和公共表表达式中无效，除非还指定了TOP或FOR XML。@seo20:尝试删除子选择中的order by，看看是否有帮助。我已经更新了我的答案。仍然收到错误：Msg 102，15级，状态1，第9行“'）附近的语法不正确。Msg 102，级别15，状态1，第27行“T1”附近的语法不正确。@seo20:缺少逗号。几乎所有works都希望UserSessionID具有TBluserSessionSequenceId中最高的UserSessionSequenceID。@seo20:您可以通过更改子选择中的
order by
子句来获得任意顺序。我已经更新了答案，给出了你想要的答案。
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