MySQL 8.0

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create table t as
    select 1 as id, 1 as userid, 0  as first,  5 as amount union all
    select 2 as id, 1 as userid, 10 as first, 10 as amount union all
    select 3 as id, 1 as userid, 10 as first,  5 as amount union all
    select 4 as id, 1 as userid, 15 as first, 10 as amount union all
    select 5 as id, 2 as userid, 0  as first, 10 as amount union all
    select 6 as id, 2 as userid, 0  as first,  5 as amount union all
    select 7 as id, 2 as userid, 10 as first,  5 as amount  
select *
from t


id
userid
first
amount


1
1
0
5

2
1
10
10

3
1
10
5

4
1
15
10

5
2
0
10

6
2
0
5

7
2
10
5


select userid, min(first), max(first + amount) as last
from (select t.*,
             sum(case when prev_last >= first then 0 else 1 end) over
                 (partition by userid order by first) as grp
      from (select t.*,
                   max(first + amount) over (partition by userid order by first range between unbounded preceding and 1 preceding) as prev_last
            from t
           ) t
     ) t
group by userid, grp;


userid
min(first)
last


1
0
5

1
10
25

2
0
15


with islands as (
      select userid, min(first) as first, max(first + amount) as last
      from (select t.*,
                   sum(case when prev_last >= first then 0 else 1 end) over
                       (partition by userid order by first) as grp
            from (select t.*,
                         max(first + amount) over (partition by userid order by first range between unbounded preceding and 1 preceding) as prev_last
                  from t
                 ) t
           ) t
      group by userid, grp
     )
select userid, sum(last - first) as total
from islands
group by userid;


userid
total


1
20

2
15