Sum Values Till the Next Appearance of a Condition - mysql

Question

Thank you for coming in.
I have a table like this:
And here is what I want to do: Segregated by id, I want to sum up the Val based on the condition.
For example, for id=1, I want the sum of Val till condition A firstly appear, then another sum of Val between the first A and the second A, and sum of Val between the second and the third A... The sum of Val when condition = B follows the same logic, but should not be influenced by A. Finally, each sum of Val only sums the same id.
How should I do this? I tried group by and partition by, but unable to obtain an ideal result. The ideal output would be like the Sum column in the picture.
Thank you very much.

Answer 1

Assuming that there is a column that specifies the ordering, then you can do what you want. SQL tables represent unordered sets. There is no ordering unless a column specifies the ordering.
You seem to want to define groups for As and Bs. You can do this using window functions. This is a little strange, because you want different groupings -- a case expression can handle that. Here is the idea:
select t.*,
(case when condition = 'A'
then sum(val) over (partition by id, grp_a order by <ordering col>)
when condition = 'B'
then sum(val) over (partition by id order by <ordering col>)
end) as calculation
from (select t.*,
sum(case when condition = 'A' then 1 else 0 end) over (partition by id order by <ordering col> desc) as grp_a
from t
) t;

Answer 2

You are looking for something like this:
WITH somatory as (SELECT id, cond, v,
SUM(v) OVER (PARTITION BY id ORDER BY id ROWS UNBOUNDED PRECEDING) as sumV
from foo),
conditional_somatory as (
select id, cond, v, sumV,
case when cond = 'A' then sumV end as somatoryA,
case when cond = 'B' then sumV end as somatoryB
from somatory
),
last_somatories as (
select id, cond, v, sumV,
SomatoryA,
max(coalesce(somatoryA,0)) over (partition by id order by id ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) as lastSomatoryA,
SomatoryB,
max(coalesce(somatoryB,0)) over (partition by id order by id ROWS BETWEEN UNBOUNDED PRECEDING AND 1 PRECEDING) as lastSomatoryB
from conditional_somatory)
select id, cond, v,
case when cond = 'A' then sumV - lastSomatoryA
when cond = 'B' then sumV - lastSomatoryB
end as somatory
from last_somatories
See working fiddle.
https://dbfiddle.uk/?rdbms=mysql_8.0&fiddle=2b2d23b1421aa8fb45aa82ed8bad8b32