我有以下表格:
tblPerson:
PersonID | Name
---------------------
1 | John Smith
2 | Jane Doe
3 | David Hoshi
表位置:
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;
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心最新的位置记录。因此,我们只对以下行真正感兴趣:LocationID 41、43 和 44。
问题是:我们如何查询这些表以提供每个人的最新数据?需要进行哪些特殊分组才能产生所需的结果?
最佳答案
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 ROW_NUMBER 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
关于sql-server - 检索查询中的最新记录,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/3182436/