create table TEST_TRG 
( 
  a1 NUMBER not null, 
  a2 VARCHAR2(100), 
  a3 TIMESTAMP(6), 
  a4 DATE 
)

CREATE OR REPLACE TRIGGER TEST_COMPOUND 
  FOR INSERT OR UPDATE ON TEST_TRG 
  COMPOUND TRIGGER 
 
  v_num  number; 
  v_time timestamp; 
  v_date date; 
 
   procedure set_defaults is 
   begin 
     if INSERTING then 
       if :new.a1 is null then 
         :new.a1 := v_num; 
       end if; 
       if :new.a3 is null then 
         :new.a3 := v_time; 
       end if;  
       if :new.a4 is null then 
         :new.a4 := v_date; 
       end if; 
     end if; 
  end; 
 
  --Executed before DML statement  BEFORE STATEMENT IS 
  BEFORE STATEMENT IS 
  BEGIN 
    v_time := systimestamp; 
    v_date := sysdate; 
  END BEFORE STATEMENT; 
 
  --Executed before each row change- :NEW, :OLD are available 
  BEFORE EACH ROW IS 
  BEGIN 
 
    if inserting then 
      select nvl(max(a1), 0) + 1 into v_num from TEST_TRG; 
    end if;   
    if updating   then 
      v_num := :old.a1; 
    end if; 
    set_defaults; 
     
  END BEFORE EACH ROW; 
 
 
END; 

insert into test_trg values(1,'Y',null,null);