Oracle 11g Release 2

By using db<>fiddle, you agree to license everything you submit by Creative Commons CC0.
create table mail_address_book (pk_serial_no number primary key, address_a varchar2(5) unique, address_b varchar2(5) unique);
create table hack_table (address varchar2(5) primary key);
create trigger hack_trigger
before insert or update or delete on mail_address_book
for each row
begin
  if inserting then
    if :new.address_a is not null then
      insert into hack_table (address) values (:new.address_a);
    end if;
    if :new.address_b is not null then
      insert into hack_table (address) values (:new.address_b);
    end if;
  elsif updating then
    -- maybe skip this is column values have swapped
    if :old.address_a is null and :new.address_a is not null then
      insert into hack_table (address) values (:new.address_a);
    elsif :old.address_a is not null and :new.address_a is null then
      delete from hack_table where address = :old.address_a;
    elsif :new.address_a != :old.address_b then
      update hack_table set address = :new.address_a where address = :old.address_a;
    end if;

    if :old.address_b is null and :new.address_b is not null then
      insert into hack_table (address) values (:new.address_b);
    elsif :old.address_b is not null and :new.address_b is null then
      delete from hack_table where address = :old.address_b;
    elsif :new.address_b != :old.address_a then
      update hack_table set address = :new.address_b where address = :old.address_b;
    end if;
  else
    delete from hack_table where address in (:old.address_a, :old.address_b);
  end if;
end;
/
insert into mail_address_book (pk_serial_no, address_a, address_b) values (1,'A','B');
1 rows affected

insert into mail_address_book (pk_serial_no, address_a, address_b) values (2,'B','A');
ORA-00001: unique constraint (FIDDLE_YUSSYYWDMFFTYEKSNISD.SYS_C00140894) violated
ORA-06512: at "FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER", line 4
ORA-04088: error during execution of trigger 'FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER'

insert into mail_address_book (pk_serial_no, address_a, address_b) values (3,'C','A');
ORA-00001: unique constraint (FIDDLE_YUSSYYWDMFFTYEKSNISD.SYS_C00140894) violated
ORA-06512: at "FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER", line 7
ORA-04088: error during execution of trigger 'FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER'

insert into mail_address_book (pk_serial_no, address_a, address_b) values (4,'C','C');
ORA-00001: unique constraint (FIDDLE_YUSSYYWDMFFTYEKSNISD.SYS_C00140894) violated
ORA-06512: at "FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER", line 7
ORA-04088: error during execution of trigger 'FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER'

insert into mail_address_book (pk_serial_no, address_a, address_b) values (5,'C',null);
1 rows affected

insert into mail_address_book (pk_serial_no, address_a, address_b) values (6,'D','C');
ORA-00001: unique constraint (FIDDLE_YUSSYYWDMFFTYEKSNISD.SYS_C00140894) violated
ORA-06512: at "FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER", line 7
ORA-04088: error during execution of trigger 'FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER'

insert into mail_address_book (pk_serial_no, address_a, address_b) values (7,'C','D');
ORA-00001: unique constraint (FIDDLE_YUSSYYWDMFFTYEKSNISD.SYS_C00140894) violated
ORA-06512: at "FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER", line 4
ORA-04088: error during execution of trigger 'FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER'

insert into mail_address_book (pk_serial_no, address_a, address_b) values (8,'D','E');
1 rows affected

insert into mail_address_book (pk_serial_no, address_a, address_b) values (9,'E','F');
ORA-00001: unique constraint (FIDDLE_YUSSYYWDMFFTYEKSNISD.SYS_C00140894) violated
ORA-06512: at "FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER", line 4
ORA-04088: error during execution of trigger 'FIDDLE_YUSSYYWDMFFTYEKSNISD.HACK_TRIGGER'

select * from mail_address_book;


PK_SERIAL_NO
ADDRESS_A
ADDRESS_B


1
A
B

5
C
null

8
D
E