SQL选择逐行增加

Question

假设我有一个列（大卫·，RunningTotal）的表：

DayId RunningTotal 
--------------------- 
1  25 
3  50 
6  100 
9  200 
10  250 

如何选择的大卫·和RunningTotal已经从量增加前一天？即如何选择：

DayId DayTotal 
--------------------- 
1  25 
3  25 
6  50 
9  100 
10  50 

我所知道的唯一的电流的方法是使用while循环我试图分解出。另外，DayId没有规则规则，只是它是一个递增的整数值，但是它增加了不规则的数量，如示例表中所示。

编辑：使用MS SQL Server 2005的

Answer 1

with cte as (
    select dayid, runningtotal, row_number() over (order by dayid asc) as row_index 
    from #the_table 
) 
select cur.dayid, cur.runningtotal - coalesce(prev.runningtotal, 0) as daytotal 
from cte cur 
    left join cte prev on prev.row_index = cur.row_index - 1 

（我真希望他们能为lead实施支持和lag函数在SQL Server中：|）

Answer 2

有可能是比这更简洁的方式，但尝试：

select t3.DayId, 
    case when t4.DayId is null then t3.RunningTotal else t3.RunningTotal - t4.RunningTotal end as DayTotal 
from (
    select t1.DayId, max(t2.DayId) as PreviousDayId as 
    from MyTable t1 
    left outer join MyTable t2 on t2.DayId < t1.DayId 
    group by t1.DayId  
) a 
inner join MyTable t3 on a.DayId = t3.DayId 
left outer join MyTable t4 on a.PreviousDayId = t4.DayId