sql problem more difficult to appear in the forum: 32 (row_number function + sql subquery cycle taking the difference)

Original: sql problem more difficult to appear in the forum: 32 (row_number function + sql subquery cycle taking the difference)

So, I feel the need to be recorded, so that after the encounter this problem again, and get answers from the idea.

sql cycle taking the difference, how to write?

 http://bbs.csdn.net/topics/390636438?page=1#post-396012416

There are a set of conditions and a number (such as 10)
can detect a plurality of data depending on conditions such as the appearance is so
ID NumOut the Num
. 1. 4 0
2. 5 0
. 3 0. 8
. 4. 6 0 ......

Num subtracting the numbers in order with each record, enough to reduce a difference NumOut assigned, not reduced, the same assignment NumOut Num, then a Save the remaining number.
In other words, the above data is updated to:
ID the Num NumOut
1 4 4
2 5 5
3 1 8
4 6 0 ......
This should be how to express sql statement? ? ?

According to my ideas should write cycles judgment, but never written cycle sql, check out the data, how you should save it? Urgently, thank advise it! ! !

Here is my solution:

  
  
--drop table tb

create TABLE tb ( ID int, Num int,NumOut int)
INSERT into tb
SELECT 1, 4, 0 UNION ALL
SELECT 2, 5, 0 UNION ALL
SELECT 3, 8, 0 UNION ALL
SELECT 4, 6, 0
go


--你要减的数
declare @ num int

set @ num = 10

;with t
as
(
select *,
row_number() over( order by @@servername) as rownum
from tb
),

tt
as
(
select id,
num,
numout,
( select sum( num) from t t2
where t2.rownum <= t1.rownum) as sum_num
from t t1
)

--更新
update tt
set numout = case when sum_num <= @ num
then num
when sum_num > @ num and
@ num - (sum_num - num) >= 0
then @ num - (sum_num - num)
else 0
end


--查询
select * from tb
/*
ID Num NumOut
1 4 4
2 5 5
3 8 1
4 6 0
*/