CREATE TABLE tablea (
    id NUMBER(4) PRIMARY KEY,
    color VARCHAR(10));

CREATE TABLE tableb (
    id NUMBER(4) PRIMARY KEY,
    aid NUMBER(4),
    TYPE VARCHAR(10));

INSERT INTO tablea VALUES(1, NULL);
INSERT INTO tablea VALUES(2, NULL);
INSERT INTO tablea VALUES(3, NULL);
INSERT INTO tablea VALUES(4, NULL);

INSERT INTO tableb VALUES(1, 1, 'Green');
INSERT INTO tableb VALUES(2, 4, 'Yellow');

COMMIT;

SELECT * FROM tablea;

/* outputs:

ID                     COLOR
---------------------- ----------
1
2
3
4                                 */

SELECT * FROM tableb;

/* outputs:

ID                     AID                    TYPE
---------------------- ---------------------- ----------
1                      1                      Green
2                      4                      Yellow     */

UPDATE tablea a
   SET a.color = (SELECT b.TYPE
                    FROM tableb b
                   WHERE b.aid = a.id);

SELECT * FROM tablea;

/* outputs:

ID                     COLOR
---------------------- ----------
1                      Green
2
3
4                      Yellow     */