How do you calculate the difference in rows of a MySQL SELECT statement?

Calculating the Difference in Rows of a MySQL SELECT Statement

In MySQL, computing the difference between rows of a SELECT statement can be accomplished through various methods. One common approach is utilizing a self-join, enabling you to compare rows within the same table. Consider the following scenario:

Problem Statement:

You have a table containing data on vehicles, including their kilometers, date, car ID, and driver. However, due to varying data entry sequences, the data may be unsorted. You aim to create a view that displays the number of kilometers driven since the last entry for each car-driver combination, resulting in the following format:

ID | Kilometers | date       | car_id | car_driver | number_km_since_last_date   
 1 | 100        | 2012-05-04 | 1 | 1 | 0  
 2 | 200        | 2012-05-08 | 1 | 1 | 100  
 4 | 600        | 2012-05-16 | 1 | 1 | 400  
 3 | 1000       | 2012-05-25 | 1 | 1 | 400

Solution 1: Join with Subquery

You can use a subquery within an INNER JOIN to retrieve the previous row's kilometers for each record:

SELECT
    mt1.ID,
    mt1.Kilometers,
    mt1.date,
    mt1.Kilometers - IFNULL(mt2.Kilometers, 0) AS number_km_since_last_date   
FROM
    myTable mt1
    LEFT JOIN myTable mt2
        ON mt2.Date = (
            SELECT MAX(Date)
            FROM myTable mt3
            WHERE mt3.Date < mt1.Date
        )
ORDER BY mt1.date

In this query, a subquery selects the maximum date before the current row's date, and this maximum date is then matched to a record in the LEFT JOIN.

Solution 2: Emulating Lag Function

MySQL does not offer a built-in lag() function, but you can imitate its behavior through a user-defined variable:

SET @kilo=0;

SELECT
    mt1.ID,
    mt1.Kilometers - @kilo AS number_km_since_last_date,
    @kilo := mt1.Kilometers Kilometers,
    mt1.date
FROM myTable mt1
ORDER BY mt1.date

In this query, the user-defined variable @kilo stores the previous row's kilometers. With each iteration, the user-defined variable is updated, effectively tracking the change in kilometers.

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