SQL> CREATE TABLE t1 (col1 NUMBER, col2 NUMBER);
SQL> INSERT INTO t1 VALUES (1,230);
SQL> INSERT INTO t1 VALUES (2,TO_NUMBER(NULL));
SQL> INSERT INTO t1 VALUES (3,400);
SQL> INSERT INTO t1 VALUES (4,300);
SQL> INSERT INTO t1 VALUES (5,700);
  
SQL> CREATE TABLE t2 (col1 NUMBER, col3 NUMBER);
SQL> INSERT INTO t2 VALUES (1,2);
SQL> INSERT INTO t2 VALUES (2,TO_NUMBER(NULL));
SQL> INSERT INTO t2 VALUES (3,4);
SQL> INSERT INTO t2 VALUES (4,3);
SQL> INSERT INTO t2 VALUES (5,7);
SQL> COMMIT;
   
SQL> CREATE OR REPLACE TRIGGER babu_fk_trigger
  2      AFTER UPDATE ON scott.t1
  3      FOR EACH ROW
  4  BEGIN
  5      IF :new.col2 IS NOT NULL
  6      AND (:old.col2 IS NULL OR :new.col2 != :old.col2) THEN
  7          UPDATE t2
  8          SET    t2.col3 = :new.col2
  9          WHERE  t2.col1 = :new.col1;
 10      END IF;
 11  END;
 12  /
  
Trigger created.
  
SQL> UPDATE t1 SET col2 = 999 WHERE col1 = 2;
  
1 row updated.
  
SQL> COMMIT;
  
Commit complete.
  
SQL> SELECT * FROM t2;
  
      COL1       COL3
---------- ----------
         1          2
         2        999
         3          4
         4          3
         5          7
  
SQL>