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我在使用 select 语句从“下一个”行检索信息时遇到问题。我在 MySQL 中有一个带有状态信息（开或关）的表，示例记录如下：

状态表：（id 为自增）

+-----+----------+----------+---------------------+
| id  | asset_id | state_id | created             |
+-----+----------+----------+---------------------+
| 917 |       97 |        1 | 2013-10-13 22:07:03 |
| 918 |       97 |        0 | 2013-10-13 22:09:34 |
| 919 |       97 |        1 | 2013-10-13 22:09:52 |
| 924 |       97 |        0 | 2013-10-14 10:39:24 |
| 939 |       97 |        1 | 2013-10-14 12:47:26 |
| 940 |       97 |        0 | 2013-10-14 12:47:27 |
+-----+----------+----------+---------------------+

我需要做的是提出一个语句，为我提供给定日期内每个资产的状态转换开始和结束时间，如下所示：结束时间始终是下一个状态的开始时间。如果一天中的最后一个状态一直持续到第二天，我会将其报告为结束时间：

+----------+---------------------+---------------------+
| state_id | starttime           | endtime             |
+----------+---------------------+---------------------+
|        1 | 2013-10-13 22:07:03 | 2013-10-13 22:09:34 |
|        0 | 2013-10-13 22:09:34 | 2013-10-13 22:09:52 |
|        1 | 2013-10-13 22:09:52 | 2013-10-14 10:39:24 |
+----------+---------------------+---------------------+

这是我目前的声明：

SELECT t1.state_id, t1.created as starttime, t2.created as endtime 
FROM state t1, state t2 
WHERE t1.created like "2013-10-13%" and t1.asset_id=97 and t2.created = 
(SELECT min(created) FROM state 
    WHERE id > t1.id and asset_id = 97 order by created desc);

当请求的日期不是结果集的最后一天时，这可以正常工作，例如“2013-10-13”的示例。问题是最后一天和最后一个状态（最后一个记录）。如果我运行 2013-10-14 的示例数据，我只会得到两个状态转换，而不是三个：

+----------+---------------------+---------------------+
| state_id | starttime           | endtime             |
+----------+---------------------+---------------------+
|        0 | 2013-10-14 10:39:24 | 2013-10-14 12:47:26 |
|        1 | 2013-10-14 12:47:26 | 2013-10-14 12:47:27 |
+----------+---------------------+---------------------+

最后一个被删除大概是因为子查询的 where 子句 'id > t1.id'。创建的记录 '2013-10-14 12:47:27' 是表中的最后一条记录，之后没有其他记录可从中获取结束时间。

我的问题是：是否可以更改查询，以便它还选择最后一个转换，例如使用 NULL 作为结束时间，如下所示：

+----------+---------------------+---------------------+
| state_id | starttime           | endtime             |
+----------+---------------------+---------------------+
|        0 | 2013-10-14 10:39:24 | 2013-10-14 12:47:26 |
|        1 | 2013-10-14 12:47:26 | 2013-10-14 12:47:27 |
|        0 | 2013-10-14 12:47:27 | NULL                |
+----------+---------------------+---------------------+
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2 回答 2

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改变

and t2.created = 
(SELECT min(created) FROM state 
    WHERE id > t1.id and asset_id = 97 order by created desc);

到

and (t2.created = 
(SELECT min(created) FROM state 
    WHERE id > t1.id and asset_id = 97 order by created desc))
    or t2.created is NULL;

这将起作用，除非有其他条件会导致 t2.created 为 NULL 并且您不希望该记录 - 没有架构很难确定。

于 2013-10-18T13:25:13.993 回答
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你需要一个左连接。试试这个。你可以在这里看到它：http ://sqlfiddle.com/#!2/c9cb7/2

SELECT t1.state_id, t1.created as starttime, t2.created as endtime 
FROM state t1
LEFT JOIN state t2 
  ON t1.asset_id = t2.asset_id
   AND t1.created < t2.created
WHERE t1.created like "2013-10-13%" and t1.asset_id=97 
  AND NOT EXISTS
    (SELECT *
     FROM state
     WHERE state.created > t1.created
      AND state.created < t2.created)
ORDER BY starttime
于 2013-10-18T16:41:11.167 回答