update Sale
set bounds=b.bounds
from 
(select ID,sum(amount*price*0.05) as bounds from product group by ID)b
where Sale.ID=a.ID

------解决方案--------------------
SQL code
update
 set bounds=b.bounds
from 
 Sale a join
(select ID,sum(amount*price*0.05) as bounds from product group by ID)b
on  
 a.ID=a.ID