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Calculating Differences between Rows in a MySQL SELECT Query
To calculate the difference between rows in a SELECT query, consider the following approach:
Using a LEFT JOIN with a MAX() Subquery
You can use a LEFT JOIN to connect a row with its previous one, based on a condition. In your case, you want to find the previous row for each date:
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
Using a Lag() Function Emulation
Since MySQL doesn't have a lag() function, you can emulate one using 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
Results
Both methods will produce the desired output:
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
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