Hello,
On a related note, would it be possible to have a "with" statement inside the subquery, .i.e. "update t1 set  data = *(with tmp_table as (select ...) select ... from ... where ...)* where exists (select * from t2 where t1.id=t2.id);"?

According to my tests it's not possible. What would be the best way to simulate it in MonetDB? Using a View/Temporary Table?

Thank you,
George

 
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);