从电读数mysql计算使用的Kwh答案

【问题标题】：Calculating Kwh used from electric readings mysql从电读数mysql计算使用的Kwh
【发布时间】：2015-03-15 18:56:50
【问题描述】：

consumer_id reading_date  previous           present         KWH used

10            January       0                 30                   30
10            February     30                 60                   30
10            March        60                 70                   10
12            January       0                 30                   30
12            February     30                 30                   15

消费表

CREATE TABLE IF NOT EXISTS `consumption` (
`consumption_id` int(20) NOT NULL,
  `reading_date` date NOT NULL,
  `readings` int(20) NOT NULL,
  `kwh_used` int(20) NOT NULL,
  `meter_reader_id` int(20) NOT NULL,
  `consumer_id` int(10) NOT NULL
) ENGINE=InnoDB AUTO_INCREMENT=35 DEFAULT CHARSET=latin1;

INSERT INTO `consumption` (`consumption_id`, `reading_date`, `readings`, `kwh_used`, `meter_reader_id`, `consumer_id`) VALUES
(14, '2013-07-30', 14386, 0, 1, 10),
(20, '2013-08-03', 14390, 0, 1, 10),
(29, '2013-07-30', 144, 0, 1, 12),
(31, '2013-07-31', 144, 0, 1, 12),
(34, '2013-08-03', 144, 0, 1, 12);

请在下面查看我的查询，了解如何获得与上面发布的第一个表相同的输出。

SET @curRank1 = 0;
SET @curRank2 = 0;
SELECT
    D1.reading_date
    ,D1.consumer_id
    ,D1.readings AS previous
    ,D2.readings AS present
    ,(D2.readings - D1.readings) AS kwh_used
FROM 
( 
    SELECT M1.*, @curRank1 := @curRank1 + 1 AS rank
    FROM consumption M1
    LEFT JOIN consumption M2
    ON Date(M1.reading_date) = Date(M2.reading_date)
    AND M1.reading_date > M2.reading_date
    WHERE M2.reading_date IS NULL
) D2
LEFT JOIN
(
    SELECT M1.*, @curRank2 := @curRank2 + 1 AS rank
    FROM consumption M1
    LEFT JOIN consumption M2
    ON Date(M1.reading_date) = Date(M2.reading_date)
) D1
ON D2.rank = (D1.rank + 1)  
WHERE D1.reading_date IS NOT NULL

那个查询的输出是这样的

reading_date    consumer_id     previous    present    kwh_used     
2013-07-30       10             14386       14390       4
2013-07-30       12             144         144         0
2013-08-03       10             14390       144         -14246
2013-08-03       12             144         144         0

如您所见，查询返回了一系列不好的读数，因为读数具有不同的 consumer_id。请让我知道我在这里缺少什么。

谢谢。

【问题讨论】：

我看不到所需结果集与数据集的关系！？！

标签： mysql database data-binding

【解决方案1】：

你应该添加：

and M1.consumer_id = m2.consumer_id

到您的每个子查询，类似于它们之间的join。

我认为有更简单的方法来获得你想要的东西（相关的 subuqeries 或使用变量）。但如果您想修复查询，请将consumer_id 放入连接条件中。

【讨论】：