SQL 合并两个时间戳不同的表作为索引的查询

Question

我不确定我的问题是否适用于 sql 查询。

我有以下两个 Mysql tables:

Table一个： table 每 20 秒获取一个新值

a.Time                  a.Value
2015-05-14 07:36:36     49
2015-05-14 07:36:56     49
2015-05-14 07:37:16     49
2015-05-14 07:37:36     49
2015-05-14 07:37:56     50
2015-05-14 07:38:16     50
2015-05-14 07:38:36     50
2015-05-14 07:38:56     49
2015-05-14 07:39:16     49
2015-05-14 07:39:36     49
2015-05-14 07:39:56     49
2015-05-14 07:40:16     50
2015-05-14 07:40:36     50
2015-05-14 07:40:56     50
2015-05-14 07:41:16     49
2015-05-14 07:41:36     49
2015-05-14 07:41:56     50
2015-05-14 07:42:16     50
2015-05-14 07:42:36     49
2015-05-14 07:42:56     49
2015-05-14 07:43:16     49
2015-05-14 07:43:36     49
2015-05-14 07:43:56     49
2015-05-14 07:44:06     49

Tableb table 获取值的每个变化一条新记录

b.Time              b.Value
2015-05-14 07:34:50     1500
2015-05-14 07:38:52     2000
2015-05-14 07:40:52     0
2015-05-14 07:42:31     1500

结果应该如下所示

a.Time                  a.Value     b.Time              b.Value
2015-05-14 07:36:36     49         2015-05-14 07:34:50  1500
2015-05-14 07:36:56     49         2015-05-14 07:34:50  1500
2015-05-14 07:37:16     49         2015-05-14 07:34:50  1500
2015-05-14 07:37:36     49         2015-05-14 07:34:50  1500
2015-05-14 07:37:56     50         2015-05-14 07:34:50  1500
2015-05-14 07:38:16     50         2015-05-14 07:34:50  1500
2015-05-14 07:38:36     50         2015-05-14 07:34:50  1500
2015-05-14 07:38:56     49         2015-05-14 07:38:52  2000
2015-05-14 07:39:16     49         2015-05-14 07:38:52  2000
2015-05-14 07:39:36     49         2015-05-14 07:38:52  2000
2015-05-14 07:39:56     49         2015-05-14 07:38:52  2000
2015-05-14 07:40:16     50         2015-05-14 07:38:52  2000
2015-05-14 07:40:36     50         2015-05-14 07:38:52  2000
2015-05-14 07:40:56     50         2015-05-14 07:40:52  0
2015-05-14 07:41:16     49         2015-05-14 07:40:52  0
2015-05-14 07:41:36     49         2015-05-14 07:40:52  0
2015-05-14 07:41:56     50         2015-05-14 07:40:52  0
2015-05-14 07:42:16     50         2015-05-14 07:40:52  0
2015-05-14 07:42:36     49         2015-05-14 07:42:31  1500
2015-05-14 07:42:56     49         2015-05-14 07:42:31  1500
2015-05-14 07:43:16     49         2015-05-14 07:42:31  1500
2015-05-14 07:43:36     49         2015-05-14 07:42:31  1500
2015-05-14 07:43:56     49         2015-05-14 07:42:31  1500
2015-05-14 07:44:06     49         2015-05-14 07:42:31  1500

tablea 中的时间戳应该与 tableb 中的下一个更小的时间戳！

知道查询应该是什么样子吗？

非常感谢！

Answer 1

使用按 TIME 和 LIMIT 1 排序的子查询从 B 获取一条时间小于当前 A.Time:

的顶级记录

SELECT 
a.Time,
a.Value,
(SELECT Time FROM B WHERE B.Time<=A.Time ORDER BY B.Time DESC LIMIT 1) 
     as B_Time,
(SELECT Value FROM B WHERE B.Time<=A.Time ORDER BY B.Time DESC LIMIT 1)  
     as B_Value
FROM A
ORDER BY A.Time

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