SQL Query将两个具有不同时间戳的表合并为索引

Question

我不确定我的问题是否适用于SQL查询。

我有以下两个Mysql表：

表a： 该表每20秒获得一个新值

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

表b 该表每次更改值都是新记录

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

表a中的时间戳应该是表b中的下一个较小的时间戳！

知道查询的外观吗？

非常感谢！

Answer 1

使用按TIME和LIMIT 1订购的子查询，从B获得一条最高记录，时间少于当前A.时间：

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