Calculating Differences Between Rows in a MySQL SELECT Statement
Problem:
How to determine the difference in values between two subsequent rows in a MySQL SELECT statement? The provided table contains data such as kilometers, date, and car identification. The goal is to create a view that includes an additional column indicating the number of kilometers since the previous date.
Solution:
To achieve this, a combination of LEFT JOIN and subquery is employed. The LEFT JOIN allows us to retrieve data from the myTable table, while the subquery ensures that the comparison is made with the most recent row that has a date prior to the current row. This approach effectively produces the desired difference in kilometers:
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
Alternative Solution:
Another method involves emulating the lag() function using MySQL hackiness:
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
Results:
Both methods successfully calculate the difference in kilometers between successive rows in the sorted table, resulting in a view that provides the desired additional information.
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