create table master (
     master_id number( 9 )
   , amount    number( 15 ,  4 )
   , constraint pk_master primary key (master_id)
)
/
create table detail (
     detail_id number( 9 )
   , master_id number( 9 ) not null
   , amount    number( 15 ,  4 ) not null
   , constraint pk_detail primary key (detail_id)
   , constraint fk_detail_master foreign key (master_id)
       references master(master_id) on delete cascade
)
/
create or replace package pkg_master as
  type tbl_master is table of master%rowtype index by binary_integer;
       v_master tbl_master;
  procedure update_amount;
end;
/
create or replace package body pkg_master as
  procedure update_amount as
  begin
    if v_master.count >  0  then
      for i in v_master.first..v_master.last loop
        if v_master.exists(i) then
          update master set
            amount = nvl(amount,  0 ) + v_master(i).amount
          where  1 = 1 
            and master_id = v_master(i).master_id;
        end if;
      end loop;
      v_master.delete;
    end if;
  end;
end;
/

create or replace trigger taiudr_detail
after insert or update or delete on detail
for each row
begin
  if inserting or updating then
    pkg_master.v_master(:new.master_id).master_id := :new.master_id;
    pkg_master.v_master(:new.master_id).amount :=
      nvl(pkg_master.v_master(:new.master_id).amount,  0 ) + :new.amount;
  end if;
  if updating or deleting then
    pkg_master.v_master(:old.master_id).master_id := :old.master_id;
    pkg_master.v_master(:old.master_id).amount :=
      nvl(pkg_master.v_master(:old.master_id).amount,  0 ) - :old.amount;
  end if;
end;
/
create or replace trigger taiud_detail
after insert or update or delete on detail
begin
  pkg_master.update_amount;
end;
/

SQL> insert into master(master_id) values ( 1 );

 1  row inserted

SQL> insert into detail(detail_id, master_id, amount) values ( 1 ,  1 ,  10 );

 1  row inserted

SQL> select * from master;

 MASTER_ID            AMOUNT
 ---------- -----------------
 
          1             10 . 0000 

SQL> insert into detail(detail_id, master_id, amount) values ( 2 ,  1 ,  20 );

 1  row inserted

SQL> select * from master;

 MASTER_ID            AMOUNT
 ---------- -----------------
 
          1             30 . 0000 

SQL> update detail set amount =  40  where detail_id =  1 ;

 1  row updated

SQL> select * from master;

 MASTER_ID            AMOUNT
 ---------- -----------------
 
          1             60 . 0000 

SQL> delete detail where detail_id =  2 ;

 1  row deleted

SQL> select * from master;

 MASTER_ID            AMOUNT
 ---------- -----------------
 
          1             40 . 0000