排序问题

create table T(id int, num int, st int)
insert T select 11, 5, 0
union all select 12, 3, -1
union all select 13, 5, 1
union all select 14, 12, 0

declare @sql varchar(8000)
set @sql='select '
select @sql=@sql+'NUM='+rtrim(num)+',ST='+rtrim(st)+',' from T order by num asc, st desc
select @sql=left(@sql, len(@sql)-1)
exec(@sql)

--result
NUM ST NUM ST NUM ST NUM ST
----------- ----------- ----------- ----------- ----------- ----------- ----------- -----------
3 -1 5 1 5 0 12 0

为什么呢？