Update woes in Oracle 8.1.6.3.0

Question

I'm trying to update one column in table A with a new value that lives in table B.
Both tables share the same primary keys but table B has considerably less rows (about 1000 rows) than the million rows that exist in Table A.
I tried this:
UPDATE TABLE_A A
SET a.COLUMN_TO_UPDATE= (SELECT b.COLUMN_TO_UPDATE
          FROM TABLE_B B
          WHERE a.KEY1=b.KEY1
AND a.KEY2=b.KEY2
AND a.KEY3=b.KEY3
AND a.KEY4=b.KEY4
     AND b.FLAG='A')
After I ran it, I realized it would probably update the entire table, it did. :(
What's the most efficient way to run this update?
Are there any threads that already identify the correct syntax for this situation?
Best Regards,
SETH

Answer 1

A simple solution is...
UPDATE table_a a
   SET a.column_to_update =
         (SELECT b.column_to_update
            FROM table_b b
           WHERE a.key1 = b.key1
             AND a.key2 = b.key2
             AND a.key3 = b.key3
             AND a.key4 = b.key4
             AND b.flag = 'A')
WHERE EXISTS
         (SELECT b.column_to_update
            FROM table_b b
           WHERE a.key1 = b.key1
             AND a.key2 = b.key2
             AND a.key3 = b.key3
             AND a.key4 = b.key4
             AND b.flag = 'A');