How to Calculate Energy Consumption Differences Between Consecutive Rows in MySQL by SN?

MySQL: Calculating Energy Consumption Differences Between Consecutive Rows by Serial Number (SN)

This document outlines a method to calculate the difference in energy consumption between consecutive rows in a MySQL database, grouped by serial number (SN).

Problem:

Given a table containing energy consumption data, the goal is to compute the consumption difference for each SN, representing the change between the current and preceding value for that specific SN.

Sample Data:

The following table (EnergyLog) illustrates the initial data structure:

SN	Date	Value
2380	2012-10-30 00:15:51	21.01
2380	2012-10-31 00:31:03	22.04
2380	2012-11-01 00:16:02	22.65
2380	2012-11-02 00:15:32	23.11
20100	2012-10-30 00:15:38	35.21
20100	2012-10-31 00:15:48	37.07
20100	2012-11-01 00:15:49	38.17
20100	2012-11-02 00:15:19	38.97
20103	2012-10-30 10:27:34	57.98
20103	2012-10-31 12:24:42	60.83

Desired Output:

The query should produce a table with an added Consumption column:

SN	Date	Value	Consumption
2380	2012-10-30 00:15:51	21.01	0.00
2380	2012-10-31 00:31:03	22.04	1.03
2380	2012-11-01 00:16:02	22.65	0.61
2380	2012-11-02 00:15:32	23.11	0.46
20100	2012-10-30 00:15:38	35.21	0.00
20100	2012-10-31 00:15:48	37.07	1.86
20100	2012-11-01 00:15:49	38.17	1.10
20100	2012-11-02 00:15:19	38.97	0.80
20103	2012-10-30 10:27:34	57.98	0.00
20103	2012-10-31 12:24:42	60.83	2.85

SQL Solution:

The following SQL query utilizes user-defined variables to achieve the desired result:

SELECT
    EL.SN,
    EL.Date,
    EL.Value,
    IF(@lastSN = EL.SN, EL.Value - @lastValue, 0.00) AS Consumption,
    @lastSN := EL.SN,
    @lastValue := EL.Value
FROM
    EnergyLog EL,
    (SELECT @lastSN := 0, @lastValue := 0) AS SQLVars
ORDER BY
    EL.SN,
    EL.Date;

Explanation:

• @lastSN and @lastValue: These are user-defined variables initialized to 0. @lastSN tracks the last processed SN, and @lastValue stores the last processed value.
• IF(@lastSN = EL.SN, EL.Value - @lastValue, 0.00): This conditional statement checks if the current SN is the same as the previous SN. If true, it calculates the consumption difference; otherwise, it sets the consumption to 0.00 (for the first entry of each SN).
• @lastSN := EL.SN, @lastValue := EL.Value: These assignments update the user-defined variables after processing each row.
• ORDER BY EL.SN, EL.Date: This clause ensures that the rows are processed in the correct order (by SN and then by date) for accurate difference calculations.

This approach efficiently calculates the energy consumption difference for each SN by leveraging MySQL's variable handling capabilities. The use of ORDER BY is crucial for the correctness of the results.

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