MySQL - Calculate accumulation since reset event in a Table - mysql

Question

This issue is a reference for my other question
Python solution has been done based on extract from MySQL DB (5.6.34) where original data are stored.
My question is: Is it possible to make such calculation straight in MySQL?
Just to remind:
There is 'runners' table with accumulated distance per runner and reset tags
runner startdate cum_distance reset_event
0 1 2017-04-01 100 1
1 1 2018-04-20 125 0
2 1 2018-05-25 130 1
3 2 2015-04-05 10 1
4 2 2015-10-20 20 1
5 2 2016-11-29 50 0
I would like to calculate an accumulated distance per runner since the reset point (my comments in brackets ()):
runner startdate cum_distance reset_event runner_dist_since_reset
0 1 2017-04-01 100 1 100 <-(no reset since begin)
1 1 2018-04-20 125 0 25 <-(125-100)
2 1 2018-05-25 130 1 30 <-(130-100)
3 2 2015-04-05 10 1 10 <-(no reset since begin)
4 2 2015-10-20 20 1 10 <-(20-10)
5 2 2016-11-29 50 0 30 <-(50-20)
So far I was able to calculate only differences between reset events:
SET #DistSinceReset=0;
SELECT
runner,
startdate,
reset_event,
IF(cum_distance - #DistSinceReset <0, cum_distance, cum_distance - #DistSinceReset) AS 'runner_dist_since_reset',
#DistSinceReset := cum_distance AS 'cum_distance'
FROM
runners
WHERE
reset_event = 1
GROUP BY runner, startdate

Answer 1

This answer is for MySQL 8.
The information you want is the most recent cum_distance for each user with reset_event = 1. You are using MySQL 8, so you can use window functions.
Here is one method:
select r.*,
(cum_distance - coalesce(preceding_reset_cum_distance, 0)) as runner_dist_since_reset
from (select r.*,
min(cum_distance) over (partition by runner order by preceding_reset) as preceding_reset_cum_distance
from (select r.*,
max(case when reset_event = 1 then start_date end) over
(partition by runner
order by start_date
rows between unbounded preceding and 1 preceding
) as preceding_reset
from runners r
) r
) r;