PL/SQL trigger to update another table(using primary keys) for insert on one table

Question

Scenario is,

TableA schema:

 PK(col_1, col_2, col_3), col_4, col_5
 col_1 VARCHAR2(2)
 col_2 VARCHAR(16)
 col_3 VARCHAR(3)
 col_4 NUMBER
 col_5 NUMBER

TableB schema:

 PK(col_1, col_2, col_3), col_4, col_5, col_6
 col_1 VARCHAR2(2)
 col_2 VARCHAR(16)
 col_3 VARCHAR(3)
 col_4 NUMBER
 col_5 NUMBER
 col_6 CHAR(1)
 col_7 CHAR(1)

---

Trigger update on TableB for every row insert in TableA

Primary keys for TableA and TableB has same composite keys - PK(col_1, col_2, col_3).

First 5 columns of TableA & TableB are same.

---

Question:

In Oracle DB, How to use primary key values of TableA row to update corresponding row in TableB? in below code...

CREATE OR REPLACE TRIGGER trigger_tableA
  BEFORE INSERT ON TableA
  FOR EACH ROW
DECLARE
BEGIN
  UPDATE TableB
  SET col_6 = 'Y'
  SET col_7 = 'Y'
  WHERE <use primary keys of inserted record in TableA>;
END trigger_tableA;
/

Answer 1

I'd use merge.

create or replace trigger trigger_table1
  before insert on tablea
  for each row
begin
  merge into tableb b
  using tablea a
  on (    a.col1 = b.col1
      and a.col2 = b.col2
      and a.col3 = b.col3
     )
  when matched then update set
    b.col6 = 'Y',
    b.col7 = 'Y';
  where b.col1 = :new.col1
    and b.col2 = :new.col2
    and b.col3 = :new.col3;
end;
/

[EDIT] On a second thought, maybe your update attempt even makes more sense then my merge. Though, should be correctly written as

create or replace trigger trigger_table2
  before insert on tablea
  for each row
begin
  update tableb b set
    b.col6 = 'Y',
    b.col7 = 'Y'
  where b.col1 = :new.col1
    and b.col2 = :new.col2
    and b.col3 = :new.col3;
end;
/

[EDIT #2] If you want, you can combine different options within the same trigger. You'd use IF-THEN-ELSE, e.g.

create or replace trigger trigger_table2
  before insert or update or delete on tablea
  for each row
begin
  if inserting then
     update tableb b set
       b.col6 = 'Y',
       b.col7 = 'Y'
     where b.col1 = :new.col1
       and b.col2 = :new.col2
       and b.col3 = :new.col3;
  elsif updating then
     update tableb b set
       b.col6 = 'N',
       b.col7 = 'Y'
     where b.col1 = :new.col1
       and b.col2 = :new.col2
       and b.col3 = :new.col3;
  elsif deleting then
     update tableb b set
       b.col6 = 'N',
       b.col7 = 'N'
     where b.col1 = :new.col1
       and b.col2 = :new.col2
       and b.col3 = :new.col3;
  end if;
end;
/