Question

我想减去两个值，即当前值和在每个值的当前值之前的行中减去的值，基本上是创建一个仅包含这些新值的新表，以便我可以对它们进行平均。

表结构如下：

-----------------------
id | keyTime   | millis| 
------------------------
1  | 22:00:00  | 10  | 
2  | 22:05:00  | 20  |
3  | 22:10:00  | 30  | 
4  | 22:15:00  | 40  |

我想这样减去：

id | dif    |
-------------
1  | 10-0   | 
2  | 20-10  |
3  | 30-20  | 
4  | 40-30  |

Answer 1

您可以使用相关子查询：

select t.*, (millis - coalesce(prev_millis, 0)) as diff
from (select t.*,
             (select t2.millis
              from t t2
              where t2.keyTime < t.keyTime
              order by t2.keyTime desc
              limit 1
             ) as prev_millis
      from t
     ) t;

这假设id不可靠。使用显示的id值，您可以简单地使用left join。

Answer 2

我认为左联接将是最好的解决方案（如果您的ID始终是连续的）

SELECT `t`.`id`,`t`.`millis`-COALESCE(`t2`.`millis`,0) AS `diff`
FROM `t`
LEFT JOIN `t` AS `t2` ON `t2`.`id` = `t`.`id`-1

否则，我将使用子查询

SELECT `id`,`millis` - COALESCE((SELECT `millis` FROM `t` AS `t2` WHERE `id` < `t`.`id` ORDER BY `id` DESC LIMIT 1),0) AS `diff`
FROM `t`

MySQL用之前的字段减去字段

2 个答案: