select c.contractpaymoney as 前一记录,
d.contractpaymoney as 下一条记录,
c.contractpaymoney-d.contractpaymoney as 前一记录与后一记录的差值
from
(
select rownum as num1 ,a.contractpaymoney from a
) c
left join
(
select rownum as num2 ,b.contractpaymoney from b
) d on c.num1=d.num2-1