How to Calculate Consumption Values by Subtracting from the Previous Row in MySQL?

Calculate consumption value by group in MySQL: based on the difference of the previous row

Calculating the difference between consecutive values ​​based on a grouping factor is a very common task in MySQL. Suppose you need to calculate the consumption value of each sequence number (SN) based on the previous value.

Data structure:

The sample data contains the EnergyLog table with the following schema:

<code>SN | 日期 | 值</code>

Expected results:

The goal is to extend the table to add a "consumption" column that represents the difference of each SN relative to the "value" of the previous row.

Solution:

MySQL variables provide a convenient way to achieve this. By using user-defined variables (@lastSN and @lastValue) we can keep track of the previous SN and value while iterating through the data.

-- 声明变量
SELECT
  EL.SN,
  EL.日期,
  EL.值,
  IF(@lastSN = EL.SN, EL.值 - @lastValue, 0.00) AS 消耗,
  @lastSN := EL.SN,
  @lastValue := EL.值
-- 来自表
FROM EnergyLog EL,
  (SELECT @lastSN := 0, @lastValue := 0) AS SQLVars
-- 按分组因子SN和日期排序
ORDER BY
  EL.SN,
  EL.日期;

Explanation:

The
• FROM clause declares variables (@lastSN and @lastValue) and initializes them to 0.
• The query iterates the EnergyLog table in the desired order (by SN and date).
• It compares the current SN with @lastSN. If there is a match, calculate the consumption (difference) between the current value and the previous @lastValue.
• If SN is different, assume a new group and reset consumption to 0.
The
• @lastSN and @lastValue variables will be updated to the next iteration.

Output:

SN	日期	值	消耗
2380	2012-10-30	21.01	0.00
2380	2012-10-31	22.04	1.03
2380	2012-11-01	22.65	0.61
2380	2012-11-02	23.11	0.46
20100	2012-10-30	35.21	0.00
20100	2012-10-31	37.07	1.86
20100	2012-11-01	38.17	1.10
20100	2012-11-02	38.97	0.80
20103	2012-10-30	57.98	0.00
20103	2012-10-31	60.83	2.85

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