select item_no,convert(varchar(7),make_date,120),sum(item_num) from(
select * from tb a 
where not exists(select 1 from tb where comp_id=a.comp_id and item_no=a.item_no and convert(varchar(7),make_date,120)=convert(varchar(7),a.make_date,120) and make_date>a.make_date)
)t group by item_no,convert(varchar(7),make_date,120)

------解决方案--------------------
SQL code

select a.item-no,sum(item-num) 'item-num'
from tab a
inner join
(select compid,item-no,max(Make_date) maxMake_date
from tab
where datepart(m,Make_date)=[指定的月]
group by compid,item-no) b
on a.compid=b.compid and a.item-no=b.item-no 
and a.Make_date=b.maxMake_date
group by a.item-no