游标疑难
create   table   A(col   smallint,ds   smallint)
insert   into   A
select   1,null
union   all
select   1,null
union   all
select   2,null
union   all
select   1,null
union   all
select   2,null
union   all
select   3,null
union   all
select   1,null
union   all
select   2,null
union   all
select   3,null
union   all
select   0,null
union   all
select   0,null
union   all
select   1,null
union   all
select   2,null
union   all
select   3,null
union   all
select   0,null
union   all
select   1,null
declare   @后   smallint
declare   @前   smallint
declare   cus   scroll   cursor
for
select   col   from     A
open   cus
        fetch   last   from   cus   into   @前
while   @@fetch_status=0
begin
--/**
if   @后   is   null
begin
set   @后=@前
end
--**/
--print   @前
--print   @后
update   A
set   ds=case
when   (@前=3)   and   (@后=0)   then   4  
when   (@前=3)   and   (@后=1)   then   3
when   (@前=2)   and   (@后=3)   then   0  
when   (@前=2)   and   (@后=1)   then   2  
when   (@前=1)   and   (@后=2)   then   0  
when   (@前=1)   and   (@后=1)   then   1  
when   (@前=0)   and   (@后=0)   then   0  
when   (@前=0)   and   (@后=1)   then   0     else   0   end
where   col=@前
--print   @col
fetch   relative   -1   from   cus   into   @前
end
close   cus
deallocate   cus
select   *   from   A
--------------
如何修改代码，使结果显示为：
col   ds  
1       1
1       0
2       2
1       0
2       0
3       3
1       0
2       0
3       4
0       0
0       0
1       0
2       0
3       4
0       0
1       1


------解决方案--------------------
原来并非无解

create table A(col smallint,ds smallint)
insert into A
select 1,null
union all
select 1,null
union all
select 2,null
union all
select 1,null
union all
select 2,null
union all
select 3,null
union all
select 1,null
union all
select 2,null
union all
select 3,null
union all
select 0,null
union all
select 0,null
union all
select 1,null
union all
select 2,null
union all
select 3,null
union all
select 0,null
union all
select 1,null

declare @t table(id int IDENTITY(1,1),col smallint,ds smallint)

insert @t
select * from a


declare @后 smallint
declare @前 smallint
d