Problems with correlated update

I think I am trying to do something really simple - and I apologize if this has been asked before.

I have 2 identical tables (A&b), and I want to update table A with new data from table B, only where the keys match. I have an update statement like this:

update a set (a.val)=
  (select a.val+nvl(b.val,0)
   from b
   where a.key=b.key(+));

So, you see, I want to add Val from B to Val of A, and update back to A the new values, but only for the records in B. The above statement will update all rows of A, putting nulls in where there was no matching record in B. I realize I could add to the end of the update statement a :

    WHERE a.key in (select distinct key from b)

but that is a very inefficient way to do it. (the tables really have four keys the in clause would look like this:

    Where a.key1||a.key2||a.key3||a.key4 in

       (select distinct b.key1||b.key2||b.key3||b.key4 from b)

Any help would be greatly appreciated.

 Thanks in advance!

• Hank

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