create table tb(num int,id int,va int)
insert into tb
select 1,null,1 union all
select 1,null,2 union all
select 1,null,3 union all
select 1,null,4 union all
select 1,null,5 union all
select 2,null,6 union all
select 2,null,7 union all
select 3,null,8 union all
select 3,null,9 union all
select 3,null,1 union all
select 3,null,2 union all
select 3,null,3 union all
select 4,null,4 union all
select 4,null,5 union all
select 4,null,6 union all
select 4,null,7
go

declare @cnt int
set @cnt = 3
;with ach as
(
    select *,px=row_number() over (partition by num order by getdate())
    from tb
)

select num,(px-1)/@cnt+1 as zuhao,cast(avg(va*1.) as decimal(18,2)) as a_value  --平均
from ach
group by num,(px-1)/@cnt
order by num

drop table tb

/****************

num         zuhao                a_value
----------- -------------------- ---------------------------------------
1           1                    2.00
1           2                    4.50
2           1                    6.50
3           1                    6.00
3           2                    2.50
4           1                    5.00
4           2                    7.00

(7 行受影响)