Sql server 检索查询中最近的记录

我有以下表格：

tblPerson：

PersonID | Name
---------------------
   1     | John Smith
   2     | Jane Doe
   3     | David Hoshi

t分配：

LocationID | Timestamp | PersonID | X | Y | Z | More Columns...
---------------------------------------------------------------
    40     | Jan. 1st  |     3    | 0 | 0 | 0 | More Info...
    41     | Jan. 2nd  |     1    | 1 | 1 | 0 | More Info...
    42     | Jan. 2nd  |     3    | 2 | 2 | 2 | More Info...
    43     | Jan. 3rd  |     3    | 4 | 4 | 4 | More Info...
    44     | Jan. 5th  |     2    | 0 | 0 | 0 | More Info...

我可以生成一个SQL查询，获取每个人的位置记录，如下所示：

SELECT LocationID, Timestamp, Name, X, Y, Z 
FROM tblLocation 
JOIN tblPerson 
ON tblLocation.PersonID = tblPerson.PersonID;

制作以下文件：

LocationID | Timestamp |    Name     | X | Y | Z |
--------------------------------------------------
    40     | Jan. 1st  | David Hoshi | 0 | 0 | 0 |
    41     | Jan. 2nd  | John Smith  | 1 | 1 | 0 |
    42     | Jan. 2nd  | David Hoshi | 2 | 2 | 2 |
    43     | Jan. 3rd  | David Hoshi | 4 | 4 | 4 |
    44     | Jan. 5th  | Jane Doe    | 0 | 0 | 0 |

我的问题是我们只关心最近的位置记录。因此，我们只对以下行感兴趣：locationID41、43和44

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问题是：我们如何查询这些表格，以提供最新的人均数据？需要进行哪些特殊分组才能产生所需的结果？

这是一个典型的“每组最大值”问题，几乎每天都会在堆栈溢出时出现。有很多方法可以解决这个问题，你可以通过以下方式找到示例解决方案。以下是一种在MySQL中实现此功能的方法：

SELECT
    location.LocationId,
    location.Timestamp,
    person.Name,
    location.X,
    location.Y,
    location.Z
FROM (
    SELECT
        LocationID,
        @rn := CASE WHEN @prev_PersonID = PersonID
                    THEN @rn + 1
                    ELSE 1
               END AS rn,
        @prev_PersonID := PersonID
    FROM (SELECT @prev_PersonID := NULL) vars, tblLocation
    ORDER BY PersonID, Timestamp DESC
) T1
JOIN tblLocation location ON location.LocationID = T1.LocationId
JOIN tblPerson person ON person.PersonID = location.PersonID
WHERE rn = 1

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MySQL没有排名/分析/窗口功能

SELECT tl.locationid, tl.timestamp, tp.name, X, Y, Z
  FROM tblPerson tp
  JOIN tblLocation tl ON tl.personid = tp.personid
  JOIN (SELECT t.personid,
               MAX(t.timestamp) AS max_date
          FROM tblLocation t
      GROUP BY t.personid) x ON x.personid = tl.personid
                            AND x.max_date = tl.timestamp

SQL Server 2005+和Oracle 9i+支持分析，因此您可以使用：

SELECT x.locationid, x.timestamp, x.name, x.X, x.Y, x.Z
  FROM (SELECT tl.locationid, tl.timestamp, tp.name, X, Y, Z,
               ROW_NUMBER() OVER (PARTITION BY tp.name ORDER BY tl.timestamp DESC) AS rank
          FROM tblPerson tp
          JOIN tblLocation tl ON tl.personid = tp.personid) x
WHERE x.rank = 1

使用变量获得与MySQL上的行数功能相同的功能：

SELECT x.locationid, x.timestamp, x.name, x.X, x.Y, x.Z
  FROM (SELECT tl.locationid, tl.timestamp, tp.name, X, Y, Z,
               CASE
                 WHEN @name != t.name THEN
                   @rownum := 1
                 ELSE @rownum := @rownum + 1
               END AS rank,
               @name := tp.name
          FROM tblLocation tl
          JOIN tblPerson tp ON tp.personid = tl.personid
          JOIN (SELECT @rownum := NULL, @name := '') r
      ORDER BY tp.name, tl.timestamp DESC) x
WHERE x.rank = 1

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正如@markbyers提到的，这个问题在堆栈溢出时经常出现

考虑到您的表格，我最常推荐以下解决方案：

SELECT p.*, l1.*
FROM tblPerson p
JOIN tblLocation l1 ON p.PersonID = l1.PersonID
LEFT OUTER JOIN tblLocation l2 ON p.PersonID = l2.PersonID AND 
  (l1.timestamp < l2.timestamp OR l1.timestamp = l2.timestamp AND l1.LocationId < l2.LocationId)
WHERE l2.LocationID IS NULL;

选择p.*，l1*
来自tblPerson p
在p.PersonID=l1.PersonID上加入tblLocation l1
p.PersonID=l2.PersonID和
（l1.timestamp

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要查看其他示例，请按照我添加到您的问题中的标签进行操作。

谢谢您为我添加该标签，比尔！谢谢你，天哪。我使用了SQLServer2005，并对其进行了一些修改，以产生所需的结果