create table tb
(
    id    int,
    grp nvarchar(2),
    a    int
)
insert into tb values(1,'A',100)
insert into tb values(2,'A',100)
insert into tb values(3,'A',null)
insert into tb values(4,'B',20)
insert into tb values(5,'B',null)
insert into tb values(6,'C',50)
insert into tb values(7,'C',null)
insert into tb values(8,'C',null)
insert into tb values(9,'C',50)
insert into tb values(10,'D',null)
insert into tb values(11,'D',60)
insert into tb values(12,'D',null)
insert into tb values(13,'E',300)
insert into tb values(14,'E',300)
insert into tb values(15,'E',300)



update t1 set t1.a = t2.a
from tb t1,tb t2
where t1.grp = t2.grp 
and isnull(t2.a,0) <> 0

select * from tb
/*
id,grp,a
1,A,100
2,A,100
3,A,100
4,B,20
5,B,20
6,C,50
7,C,50
8,C,50
9,C,50
10,D,60
11,D,60
12,D,60
13,E,300
14,E,300
15,E,300

(15 行受影响)

*/