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ÓÃSQLʵÏÖBn=Bn-1+An(n>=2)

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seq  a      b
1    0      100
2    -200  0
3    100    0
4    200    0
5    300    0
6    -400  0
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seq  a      b
1    0      100
2    -200  -100
3    100    0
4    200    200
5    300    500
6    -400  100
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---

select seq , a,b=a - (select sum(b) from t1 where seq < t.seq) from t1 t

---

SQL code:
use tempdb
if object_id('tb') is not null drop table tb
go
create table tb([seq] INT,[a] INT,[b] INT)
insert into tb
select 1,0,100 union all
select 2,-200,0 union all
select 3,100,0 union all
select 4,200,0 union all
select 5,300,0 union all
select 6,-400,0
go
--select * from tb

update tb set b = (select sum(t.a+t.b) from tb t where t.seq<=tb.seq)

select * from tb

/*
seq a b
----------- ----------- -----------
1 0 100
2 -200 -100
3 100 0
4 200