MySQL 8.0

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.

CREATE TABLE foo (
id INT,
cond VARCHAR(20), 
v INT);

insert into foo (id, cond, v)
values (1, '', 1), (1, 'A', 0), (1, '', 3), (1, '', 4), (1, 'B', 0), (1, '', 6), (1, 'A', 0), (1, '', 8), (1, '', 9),
(1, 'A', 0), (2, '', 11), (2, 'A', 0), (3, '', 13), (3, 'B', 0), (3, '', 15),(3, 'A', 0);
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


id
cond
v
somatory


1

1
null

1
A
0
1

1

3
null

1

4
null

1
B
0
8

1

6
null

1
A
0
13

1

8
null

1

9
null

1
A
0
17

2

11
null

2
A
0
11

3

13
null

3
B
0
13

3

15
null

3
A
0
28

id	cond	v	somatory
1		1	null
1	A	0	1
1		3	null
1		4	null
1	B	0	8
1		6	null
1	A	0	13
1		8	null
1		9	null
1	A	0	17
2		11	null
2	A	0	11
3		13	null
3	B	0	13
3		15	null
3	A	0	28