Question

我有以下表格：

tblPerson：

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

tblLocation：

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查询来获取每个Person的Location记录，如下所示：

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

产生以下内容：

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注最新的位置记录。因此，我们只对以下行感兴趣：LocationID 41,43和44。

问题是：我们如何查询这些表格，以便为每个人提供最新数据？需要什么特殊的分组才能产生预期的结果？

Answer 1

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

使用变量与MySQL上的ROW_NUMBER功能相同：

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

Answer 2

这是几乎每天都出现在Stack Overflow上的经典“每组最大”问题。有很多方法可以解决它，您可以通过searching Stack Overflow找到示例解决方案。以下是在MySQL中可以实现的一种方法：

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

Answer 3

正如@Mark Byers所提到的，Stack Overflow上经常出现这个问题。

根据您的表格，这是我最常推荐的解决方案：

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;

要查看其他示例，请按照我在问题中添加的标记greatest-n-per-group进行操作。

检索查询中的最新记录

3 个答案: