Question

我在使用select语句从'next'行检索信息时遇到问题。我在MySQL中有一个表，其中包含状态信息（打开或关闭），示例记录如下：

状态表：（id是自动增量）

+-----+----------+----------+---------------------+
| 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声明，告诉我在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 |
+----------+---------------------+---------------------+

这是我目前的陈述：

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（例如“2013-10-13”的示例），这可以正常工作。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，我只获得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 |
+----------+---------------------+---------------------+

最后一个被丢弃的大概是因为子查询的where子句'id＆gt; t1.id”。创建的记录“2013-10-14 12:47:27”是表格中的最后一个，并且在获取结束时间之后没有其他记录。

我的问题是：是否可以更改查询，以便它也选择最后一个转换，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                |
+----------+---------------------+---------------------+

Answer 1

更改

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;

除非有其他条件会导致t2.created为NULL并且您不想要该记录，否则这将无效。

Answer 2

你需要LEFT JOIN。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

用于从“下一行”中选择信息的SQL语句，最后一条记录将丢失

2 个答案: