mysql subtract two rows and places into Alias

Question

I have a table transaction with columns: date, approved. I'm trying to subtract two rows (approved) together and put the results into an alias called approved_diff

I'm using:

SELECT date, approved FROM transaction;

date	approved
2022-06-24 09:43:00	10
2022-06-24 09:45:00	15
2022-06-24 09:50:00	50
2022-06-24 09:52:00	51
2022-06-24 09:57:00	56

what I would like is:

date	approved	approved_diff
2022-06-24 09:43:00	10	10
2022-06-24 09:45:00	15	5
2022-06-24 09:50:00	16	1
2022-06-24 09:52:00	20	4
2022-06-24 09:57:00	22	2

Can someone help me?

Answer 1

On MySQL 8+, we can use the LAG() analytic function here:

SELECT date, approved,
       approved - LAG(approved, 1, 0) OVER (ORDER BY date) AS approved_diff
FROM yourTable
ORDER BY date;

On earlier versions of MySQL, we can use a correlated subquery in place of LAG() :

SELECT date, approved,
       approved - (SELECT approved FROM yourTable t2
                   WHERE t2.date < t1.date
                   ORDER BY t2.date DESC LIMIT 1) AS approved_diff
FROM yourTable t1
ORDER BY date;