UPDATE table_a
   SET column_a = column_b
 WHERE userid IN (SELECT userid
                    FROM b)

UPDATE table_a
   SET (column_1, column__2) = (SELECT column_a, column_b
                                  FROM table_b
                                 WHERE join_column_1 = table_a.join_column_a);

UPDATE table_a
   SET column_1 = (SELECT COLUMN
                     FROM table_b
                    WHERE userid = table_a.userid);

UPDATE table_a
   SET (column_1) = (SELECT COLUMN
                       FROM table_b
                      WHERE userid = table_a.userid AND url IS NOT NULL);