Subtract column values using coalesce - sqlite

Question

I want to subtract values in the "place" column for each record returned in a "race", "bib", "split" group by so that a "diff" column appears like so.
Desired Output:
race | bib | split | place | diff
----------------------------------
10 | 514 | 1 | 5 | 0
10 | 514 | 2 | 3 | 2
10 | 514 | 3 | 2 | 1
10 | 17 | 1 | 8 | 0
10 | 17 | 2 | 12 | -4
10 | 17 | 3 | 15 | -3
I'm new to using the coalesce statement and the closest I have come to the desired output is the following
select a.race,a.bib,a.split, a.place,
coalesce(a.place -
(select b.place from ranking b where b.split < a.split), a.place) as diff
from ranking a
group by race,bib, split
which produces:
race | bib | split | place | diff
----------------------------------
10 | 514 | 1 | 5 | 5
10 | 514 | 2 | 3 | 2
10 | 514 | 3 | 2 | 1
10 | 17 | 1 | 8 | 8
10 | 17 | 2 | 12 | 11
10 | 17 | 3 | 15 | 14
Thanks for looking!

Answer 1

To compute the difference, you have to look up the value in the row that has the same race and bib values, and the next-smaller split value:
SELECT race, bib, split, place,
coalesce((SELECT r2.place
FROM ranking AS r2
WHERE r2.race = ranking.race
AND r2.bib = ranling.bib
AND r2.split < ranking.split
ORDER BY r2.split DESC
LIMIT 1
) - place,
0) AS diff
FROM ranking;