Thanks for your reply and clarification!
With Jan2014 release, the two functions can be expressed in the following working forms:
1) update t1 set data=(select t1.data+t2.data from t2 where t2.id=t1.id) where exists (select * from t2 where t1.id=t2.id);
2) update t1 set data = (select sum(t2.data) from t2 where t2.id=t1.idgroup by t2.id) where exists (select * from t2 where t1.id=t2.id);