MySQL 按日期时间关联子查询顺序答案

【问题标题】：MySQL correlated subquery ordered by date&timeMySQL 按日期时间关联子查询顺序
【发布时间】：2013-04-07 16:03:04
【问题描述】：

下午所有，我是stackoverflow处女，所以请放轻松。我很难从数据库中检索我需要的信息，并相信相关的子查询可能是答案，但我不确定。

我需要的所有信息都来自一张表：

╔══════╦══════════════╦══════════════╦════════════╦══════╦═════════════════════╦═════════════╗
║ COL1 ║     COL2     ║   FILENAME   ║    COL4    ║ COL5 ║      DATETIME       ║    COL7     ║
╠══════╬══════════════╬══════════════╬════════════╬══════╬═════════════════════╬═════════════╣
║    2 ║ generaluser1 ║ shared2.txt  ║ Shared     ║ XP1  ║ 2013-04-04 19:23:09 ║ Checked In  ║
║    3 ║ generaluser1 ║ shared2.txt  ║ Shared     ║ XP1  ║ 2013-04-04 19:24:06 ║ Checked Out ║
║    4 ║ generaluser1 ║ shared2.txt  ║ Shared     ║ XP1  ║ 2013-04-04 19:24:20 ║ Checked In  ║
║    5 ║ generaluser1 ║ restdoc3.txt ║ Restricted ║ XP1  ║ 2013-04-04 19:25:04 ║ Checked In  ║
║    6 ║ generaluser1 ║ restdoc3.txt ║ Restricted ║ XP1  ║ 2013-04-04 19:26:08 ║ Checked Out ║
║    7 ║ generaluser1 ║ restdoc3.txt ║ Restricted ║ XP1  ║ 2013-04-04 19:26:21 ║ Checked In  ║
║    8 ║ generaluser1 ║ shared2.txt  ║ Shared     ║ XP1  ║ 2013-04-04 19:26:51 ║ Checked Out ║
║    9 ║ generaluser1 ║ restdoc3.txt ║ Restricted ║ XP1  ║ 2013-04-04 19:28:41 ║ Checked Out ║
║   10 ║ generaluser1 ║ restdoc3.txt ║ Restricted ║ XP1  ║ 2013-04-04 19:31:01 ║ Checked In  ║
║   11 ║ generaluser2 ║ restdoc3.txt ║ Restricted ║ XP2  ║ 2013-04-04 19:31:13 ║ Checked Out ║
║   12 ║ generaluser1 ║ shared2.txt  ║ Shared     ║ XP1  ║ 2013-04-04 19:33:03 ║ Checked In  ║
║   13 ║ generaluser2 ║ restdoc3.txt ║ Restricted ║ XP2  ║ 2013-04-04 19:33:28 ║ Checked In  ║
║   14 ║ generaluser2 ║ restdoc4.txt ║ Restricted ║ XP2  ║ 2013-04-04 19:33:36 ║ Checked In  ║
╚══════╩══════════════╩══════════════╩════════════╩══════╩═════════════════════╩═════════════╝

我要做的是为日期时间最新的每个文档名称 (col2) 检索一行。

因此，我想为上述行检索/输出的结果将是：

╔══════╦══════════════╦══════════════╦════════════╦══════╦═════════════════════╦════════════╗
║ COL1 ║     COL2     ║   FILENAME   ║    COL4    ║ COL5 ║      DATETIME       ║    COL7    ║
╠══════╬══════════════╬══════════════╬════════════╬══════╬═════════════════════╬════════════╣
║   12 ║ generaluser1 ║ shared2.txt  ║ Shared     ║ XP1  ║ 2013-04-04 19:33:03 ║ Checked In ║
║   13 ║ generaluser2 ║ restdoc3.txt ║ Restricted ║ XP2  ║ 2013-04-04 19:33:28 ║ Checked In ║
║   14 ║ generaluser2 ║ restdoc4.txt ║ Restricted ║ XP2  ║ 2013-04-04 19:33:36 ║ Checked In ║
╚══════╩══════════════╩══════════════╩════════════╩══════╩═════════════════════╩════════════╝

我可以设法获取我选择的文档或所有文档的最新行，但不能获取每个唯一文档的最新行。

提前感谢任何帮助我的人。

【问题讨论】：

标签： mysql sql greatest-n-per-group correlated-subquery

【解决方案1】：

假设包含文件名的列名是FileName，包含日期和时间的列是DateTime。

SELECT  a.*
FROM    TableName a
        INNER JOIN
        (
            SELECT  FileName, MAX(DateTime) max_date
            FROM    TableName
            GROUP   BY FileName
        ) b ON a.FileName = b.FileName AND
                a.DateTime = b.max_Date

SQLFiddle Demo

输出

╔══════╦══════════════╦══════════════╦════════════╦══════╦═════════════════════╦════════════╗
║ COL1 ║     COL2     ║   FILENAME   ║    COL4    ║ COL5 ║      DATETIME       ║    COL7    ║
╠══════╬══════════════╬══════════════╬════════════╬══════╬═════════════════════╬════════════╣
║   12 ║ generaluser1 ║ shared2.txt  ║ Shared     ║ XP1  ║ 2013-04-04 19:33:03 ║ Checked In ║
║   13 ║ generaluser2 ║ restdoc3.txt ║ Restricted ║ XP2  ║ 2013-04-04 19:33:28 ║ Checked In ║
║   14 ║ generaluser2 ║ restdoc4.txt ║ Restricted ║ XP2  ║ 2013-04-04 19:33:36 ║ Checked In ║
╚══════╩══════════════╩══════════════╩════════════╩══════╩═════════════════════╩════════════╝

【讨论】：

像梦中的 JW 一样工作。我不知道连接对单个表有任何用处。非常感谢，非常感谢。

【解决方案2】：

查询：

SQLFIDDLEexample

SELECT a.*
FROM TableName a
WHERE a.DateTime = (SELECT MAX(b.DateTime)
                    FROM TableName b
                    WHERE b.FileName = a.FileName)

结果：

| COL1 |         COL2 |     FILENAME |       COL4 | COL5 |                     DATETIME |       COL7 |
------------------------------------------------------------------------------------------------------
|   12 | generaluser1 |  shared2.txt |     Shared |  XP1 | April, 04 2013 19:33:03+0000 | Checked In |
|   13 | generaluser2 | restdoc3.txt | Restricted |  XP2 | April, 04 2013 19:33:28+0000 | Checked In |
|   14 | generaluser2 | restdoc4.txt | Restricted |  XP2 | April, 04 2013 19:33:36+0000 | Checked In |

【讨论】：