How to Calculate Kilometer Differences Between Rows in a MySQL Table?

Calculating Differences Between Rows in a MySQL SELECT Statement

In this scenario, you have a MySQL table containing data such as kilometers, dates, and car information, but it may not always be in chronological order. Your goal is to create a view that includes an additional column displaying the number of kilometers since the last date for each car.

To achieve this, you cannot rely on a simple JOIN on the ID column because the data is not sorted chronologically. Instead, you need to employ a LEFT JOIN and a subquery to determine the maximum date that precedes the current date for each row.

Using a LEFT JOIN and a Subquery

The following query uses a LEFT JOIN to connect the current table row (mt1) with a row (mt2) that has the maximum date prior to mt1's date:

This query calculates the difference between the kilometers of the current row (mt1) and the kilometers of the row with the maximum date prior to the current date (mt2). If mt2 is NULL, it means there is no previous row, so the difference is 0.

Alternative Method Using MySQL Hackery

As an alternative, you can use a MySQL hack to emulate the lag() function, which is not natively supported in MySQL:

This query uses a user-defined variable (@kilo) to keep track of the previous kilometer value. The difference between the current kilometer value and @kilo gives you the number of kilometers since the last date. The @kilo variable is updated with the current kilometer value after each row is processed.

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