使用标量子查询创建累计和

create view v(id, amt, trx)
as
select 1, 100, 'PR' from dual union all
select 2, 100, 'PR' from dual union all
select 3, 50, 'PY' from dual union all
select 4, 100, 'PR' from dual union all
select 5, 50, 'PY' from dual;

select case when v1.trx = 'PY' then 'payment' else 'purchase' end as trx_type,
v1.amt,
(select sum(case when v2.trx = 'PY' then -v2.amt else v2.amt end) from v v2 where v2.id <= v1.id) as blance 
from v v1;

TRX_TYPE        AMT     BLANCE
-------- ---------- ----------
purchase        100        100
purchase        100        200
payment          50        150
purchase        100        250
payment          50        200