select month,salary,
lag(salary,1,null) lastsalary,
lead(salary,1,null)nextsalary 
from a

------解决方案--------------------
如果表中月份是按顺序排列的话  上面可以了 不是的话 可以先排序再获取  或者使用add_mongths()来计算获取

------解决方案--------------------

探讨

SQL code

select month,salary,
lag(salary,1,null) lastsalary,
lead(salary,1,null)nextsalary 
from a


------解决方案--------------------
SQL code

SELECT MONTH,SALARY,LEAD(SALARY,1,SALARY)OVER(ORDER BY MONTH) LASTSALARY,
       LAG(SALARY,1,SALARY)OVER(ORDER BY MONTH) NEXTSALARY
  FROM( SELECT MONTH,SUM(SALARY) SALARY FROM T GROUP BY MONTH ) T1;

------解决方案--------------------

with  a
as
(
select 2006 salary,6 month from dual
union select 2007 salary,7 month from dual
union select 2008 salary,8 month from dual
)
select month,salary,
lag(salary,1,null) over(order by month) lastsalary,
lead(salary,1,null) over(order by month) nextsalary 
from a