求一条SQL update语句

有以下数据：
srn groupcode Field30
h30511 *20111207000079* LK169849491CN
h30510 *20111207000079* NULL
u70377 *20111207000116* NULL
u70374 *20111207000116* NULL
u70375 *20111207000116* LK168318165CN
u70376 *20111207000116* NULL

把groupcode相同的对应的Field30列进行更新，要得到以下结果：
srn groupcode Field30
h30511 *20111207000079* LK169849491CN
h30510 *20111207000079* LK169849491CN
u70377 *20111207000116* LK168318165CN
u70374 *20111207000116* LK168318165CN
u70375 *20111207000116* LK168318165CN
u70376 *20111207000116* LK168318165CN
.....
.....

SQL code

update a set [Field30]=(select max([Field30]) from tb where groupcode=a.groupcode)
from tb a

SQL code

update
  a
set
  field30=b.field30
from
  tb a,
  (select groupcode,max([Field30]) from tb group by groupcode)b
where
   a.groupcode=b.groupcode