update 
  a
set
  heji=b.zuoyeliang
from
  a
join
  (select zyhao,sum(zuoyeliang) as zuoyeliang from b group b y zyhao)
on
   a.zyhao=b.zyhao

------解决方案--------------------

SQL code

update 表A set heji=b.zuoyeliang
from 表A a
inner join
(select zyhao,sum(zuoyeliang) as zuoyeliang from 表B group by zyhao) b
on a.zyhao=b.zyhao

------解决方案--------------------

SQL code
--1
SELECT  B.zyhao ,
        SUM(zuoyeliang)
FROM    B
WHERE   EXISTS ( SELECT 1
                 FROM   A
                 WHERE  zyhao = B.zyhao )
GROUP BY B.zyhao

--2  
UPDATE  A
SET     A.heji = T.heji
FROM    ( SELECT    B.zyhao ,
                    SUM(zuoyeliang) AS heji
          FROM      B
          WHERE     EXISTS ( SELECT 1
                             FROM   A
                             WHERE  zyhao = B.zyhao )
          GROUP BY  B.zyhao
        ) T
WHERE   A.zyhao = T.zyhao