[SQL] subqueries as values in updates

[patrick . jacquot]

hello, list.
are subqueries alloved as values in an update command ?
e.g.

update a set a.attribute1 = a.attribute1 +
    (select sum(b.attribute1) from b where b.attribute2=a.attribute2);

If yes, how is the correct syntax ?
If not, is there a hope for it in a future version ?