Solution for the above Query

create table tab1(itemid int,itemname varchar(20) null,qty int)
create table tab2(orderid int,itemid int null,qty int)
----insert into tab1 values(1,'pepsi ',100)

create trigger tr_pepsi_sai
on tab2
for insert
as
begin
update a set a.qty=a.qty-b.qty
from tab1 a,tab2 b
end


select * from tab1

...