create table ta
(yhbh char(5), ye decimal(5,2))

insert into ta
select '001', 1.12 union all
select '002', 1.23

create table tb
(yhbh char(5), sbbh char(5), ye decimal(5,2))

insert into tb
select '001', '01', 0 union all
select '001', '02', 0 union all
select '002', '03', 0 union all
select '002', '04', 0


update tb set tb.ye=ta.ye
from tb
inner join ta on tb.yhbh=ta.yhbh
where tb.sbbh in
(select min(sbbh) from tb group by yhbh)

select * from tb

yhbh  sbbh  ye
----- ----- --------
001   01    1.12
001   02    0.00
002   03    1.23
002   04    0.00

(4 row(s) affected)