merge statement in trigger

I created table ra.test1(a number, b varchar2(10)) and table ra.test2(a number, b varchar2(10))

Try to sync two tables using merge statement in trigger

RA@mainpro2 > CREATE OR REPLACE TRIGGER ra.trai_test1

  2  AFTER insert ON ra.test1

  3  FOR EACH ROW

  4  DECLARE

  5 

  6 

  7  v_a number;

  8  v_b varchar2(10);

  9 

10  BEGIN

11 

12  v_a := :new.a;

13  v_b := :new.b;

14 

15   merge INTO test2 t

16       USING ( select v_a, v_b from dual

17        ) s

18       ON ( s.v_a = t.a)

19       WHEN matched THEN UPDATE SET

20         t.a  = s.v_a

21         , t.b  = s.v_b

22       WHEN NOT MATCHED THEN INSERT

23       (

24         a