SQL 按选择分组

Question

我正在使用 SQL Server，并且我有一个包含以下列的表:

SessionId | Date | first name | last name 

我想要按 sessionId 进行分组，然后获取具有最大日期的行。

例如:

xxx | 21/12/2012 | f1 | l1
xxx | 20/12/2012 | f2 | l2
yyy | 21/12/2012 | f3 | l3
yyy | 20/12/2012 | f4 | l4

我想获得以下行:

xxx | 21/12/2012 | f1 | l1
yyy | 21/12/2012 | f3 | l3

谢谢

Answer 1

试试这个:

WITH MAXSessions
AS
(
  SELECT 
    *,
    ROW_NUMBER() OVER(PARTITION BY SessionID ORDER BY Date DESC) rownum
  FROM Sessions
)
SELECT
  SessionId,
  Date,
  firstname,
  lastname 
FROM MAXSessions
WHERE rownum = 1;

或者:

SELECT 
  s.SessionId,
  s.Date,
  s.firstname,
  s.lastname 
FROM Sessions s
INNER JOIN
(
   SELECT SessionID, MAX(Date) LatestDate
   FROM sessions
   GROUP BY SessionID
) MAxs  ON maxs.SessionID  = s.SessionID
       AND maxs.LatestDate = s.Date;

<小时/>

更新:要获取 session 计数，您可以执行以下操作:

SELECT 
  s.SessionId,
  s.Date,
  s.firstname,
  s.lastname,
  maxs.SessionsCount
FROM Sessions s
INNER JOIN
(
   SELECT SessionID, COUNT(SessionID), SessionsCount, MAX(Date) LatestDate
   FROM sessions
   GROUP BY SessionID
) MAxs  ON maxs.SessionID  = s.SessionID
       AND maxs.LatestDate = s.Date;