sql利用rownum求相邻记录差值

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