--try
update p_data
    set abcclass=(select abcclass from p_data a where p_data.productcode=a.productcode and  a.year=2012 and a.month=3)
    where p_data.year=2012 and p_data.month=7 and p_data.abcclass is null;

------解决方案--------------------
一楼正解

------解决方案--------------------

update p_data
   set abcclass=(select abcclass from p_data a where p_data.productcode=a.productcode and  a.year=2012 and a.month=3)
   where p_data.year=2012 and p_data.month=7 and p_data.abcclass is null;


------解决方案--------------------
update p_data a
 set abcclass=(select abcclass from p_data a where p_data.productcode=a.productcode and a.year=2012 and a.month=3) b
 where a.year=2012 and a.month=7 and a.abcclass is null and a.id = b.id;

上面的ID都是你查询出来的同一条记录的值。