SQL> update table_a
     set field_a2 = (select filed_b2 from table_a where field_a1 = table_b.field_b1)
     where exists (select null from table_a where field_a1 = table_b.field_b1);

SQL> CREATE TABLE TEST_V003 ( EMP_ID VARCHAR2(3) primary key, SAL NUMBER(8,2));

Table created.

SQL> INSERT INTO TEST_V001 VALUES ('&A',&B);
Enter value for a: 1
Enter value for b: 8000
old   1: INSERT INTO TEST_V001 VALUES ('&A',&B)
new   1: INSERT INTO TEST_V001 VALUES ('1',8000)

1 row created.

SQL> /
Enter value for a: 2
Enter value for b: 9000
old   1: INSERT INTO TEST_V001 VALUES ('&A',&B)
new   1: INSERT INTO TEST_V001 VALUES ('2',9000)

1 row created.

SQL> COMMIT;

Commit complete.

SQL>  CREATE TABLE TEST_V002 ( ID VARCHAR2(3) primary key, SAL NUMBER(8,2));

Table created.

SQL> INSERT INTO TEST_V002 VALUES ('&A',&B);
Enter value for a: 1
Enter value for b: 4000
old   1: INSERT INTO TEST_V002 VALUES ('&A',&B)
new   1: INSERT INTO TEST_V002 VALUES ('1',4000)

1 row created.

SQL> /
Enter value for a: 2
Enter value for b: 9999
old   1: INSERT INTO TEST_V002 VALUES ('&A',&B)
new   1: INSERT INTO TEST_V002 VALUES ('2',9999)

1 row created.

SQL> /
Enter value for a: 3
Enter value for b: 9998
old   1: INSERT INTO TEST_V002 VALUES ('&A',&B)
new   1: INSERT INTO TEST_V002 VALUES ('3',9998)

1 row created.

SQL> COMMIT;

Commit complete.

SQL> SELECT * FROM TEST_V001
  2  ;

EMP        SAL
--- ----------
1         8000
2         9000

SQL> ED
Wrote file afiedt.buf

  1* SELECT * FROM TEST_V002
  2  /

ID         SAL
--- ----------
1         4000
2         9999
3         9998


SQL> ed
Wrote file afiedt.buf

  1  update test_v002
  2       set sal = (select sal from test_v001 where emp_id = test_v002.id)
  3*      where exists (select null from test_v001 where emp_id = test_v002.id)
SQL> /

2 rows updated.

SQL> select * from test_v002;

ID         SAL
--- ----------
1         8000
2         9000
3         9998