/*原始数据*/
drop table test_tab purge;

create table test_tab as
(
select 1 as seqno,50 as amount from dual
union all
select 2 as seqno,-100 as amount from dual
union all
select 3 as seqno,20 as amount from dual
union all
select 4 as seqno,50 as amount from dual
union all
select 5 as seqno,-20 as amount from dual
union all
select 6 as seqno,-30 as amount from dual
union all
select 7 as seqno,40 as amount from dual
union all
select 8 as seqno,100 as amount from dual
union all
select 9 as seqno,20 as amount from dual
union all
select 10 as seqno,-70 as amount from dual
union all
select 11 as seqno,40 as amount from dual
);

要求
按序号seqno累计
上月累计金额＝amount＋上月累计金额
但是如果上月累计金额是正值，就不计入下个月，重新累计，结果如下：

SEQNO  AMOUNT      TOT_AMOUNT
1      50          50
2      -100        -100
3      20          -80
4      50          -30
5      -20         -50
6      -30         -80
7      40          -40
8      100         60
9      20          20
10     -70         -70
11     40          -30