Who can help me with a trigger?

Question

Hi, I've got a little problem with my database that can be solved, or so I think, with a trigger. First of all, here it is.
create table utente
(id_utente number(5) Primary Key,
nome varchar2(40),
sexo varchar2(1) check (sexo='M' or sexo='F'),
morada varchar2(60),
     data_nascimento date,
     contacto number(10),
     numero_BI number(9)
create table servico
(id_servico number(5) Primary Key,
     descricao varchar2(40),
     valor number(5),
     data_inicio date,
data_fim date,
check (data_fim>=data_inicio)
create table modalidade
(id_modalidade number(5) Primary Key,
     descricao varchar2(40),
     valor number(5)
create table quarto
(id_quarto number(5) Primary Key,
     descricao varchar2(40)
create table inscricao_servico
(id_inscricao_servico number(5) Primary Key,
     id_servico number(5) references servico(id_servico),
     id_utente number(5) references utente(id_utente),
     data_pagamento date
create table inscricao_mod
(id_inscricao_mod number(5) Primary Key,
     id_modalidade number(5) references modalidade(id_modalidade),
     id_utente number(5) references utente(id_utente),
     data_inscricao date,
     data_cessacao date,
check (data_cessacao>data_inscricao)
create table estadia
(id_estadia number(5) Primary Key,
     id_utente number(5) references utente(id_utente),
     id_quarto number(5) references quarto(id_quarto),
     data_entrada date,
     data_saida date,
check (data_saida>data_entrada)
create table pag_mensalidade
     (id_pag_mensalidade number(5) Primary Key,
     id_inscricao_mod number(5) references inscricao_mod(id_inscricao_mod),
     mes_mensalidade number(6),
     data_pagamento date,
     unique (id_inscricao_mod,mes_mensalidade)
insert into utente values (1, 'Manel', 'M', 'Rua grande', to_date('80.02.02','yy.mm.dd'), 123456789, 687654321);
insert into utente values (2, 'Artur', 'M', 'Rua pequena', to_date('81.03.04', 'yy.mm.dd'), 223456789, 587654321);
insert into utente values (3, 'Cardoso', 'M', 'Rua estreita', to_date('82.04.05', 'yy.mm.dd'), 323456789, 487654321);
insert into utente values (4, 'Abílio', 'M', 'Rua larga', to_date('79.07.06', 'yy.mm.dd'), 423456789, 787654321);
insert into utente values (5, 'Antunes', 'M', 'Rua nova', to_date('79.06.01', 'yy.mm.dd'), 423557982, 387654822);
insert into modalidade values (1, 'Total', 800);
insert into modalidade values (2, 'Diurno ate 18', 600);
insert into modalidade values (3, 'Diurno ate 22', 700);
insert into quarto values (1, '412');
insert into quarto values (2, '413');
insert into quarto values (3, '414');
insert into quarto values (4, '415');
insert into quarto values (5, '416');
insert into servico values (1, 'Excursao a Fatima', 150, to_date('11.05.13', 'yy.mm.dd'), to_date('11.05.13', 'yy.mm.dd'));
insert into servico values (2, 'Ida ao cinema', 10, to_date('11.08.22', 'yy.mm.dd'), to_date('11.08.22', 'yy.mm.dd'));
insert into servico values (3, 'Apoio domiciliario', 100, to_date('11.07.01', 'yy.mm.dd'), to_date('11.07.31', 'yy.mm.dd'));
insert into estadia values (1, 1, 1, to_date('11.05.05', 'yy.mm.dd'), null);
insert into estadia values (2, 2, 2, to_date('10.01.02', 'yy.mm.dd'), null);
insert into estadia values (3, 3, 3, to_date('09.12.15', 'yy.mm.dd'), to_date('10.02.25', 'yy.mm.dd'));
insert into inscricao_mod values (1, 1, 1, to_date('09.12.15', 'yy.mm.dd'), null);
insert into inscricao_mod values (2, 2, 2, to_date('11.11.11', 'yy.mm.dd'), null);
insert into inscricao_mod values (3, 1, 3, to_date('10.04.10', 'yy.mm.dd'), to_date('11.09.21', 'yy.mm.dd'));
insert into inscricao_servico values (1, 1, 1, null);
insert into inscricao_servico values (2, 1, 2, to_date('11.10.01', 'yy.mm.dd'));
insert into inscricao_servico values (3, 2, 4, null);
insert into inscricao_servico values (4, 3, 5, to_date('10.12.12', 'yy.mm.dd'));
insert into pag_mensalidade values (1, 1, 201110, to_date('11.10.23', 'yy.mm.dd'));
insert into pag_mensalidade values (2, 2, 201111, to_date('11.11.27', 'yy.mm.dd'));
insert into pag_mensalidade values (3, 3, 201112, NULL);
insert into pag_mensalidade values (4, 1, 201111, NULL);
drop table estadia;
drop table inscricao_servico;
drop table quarto;
drop table pag_mensalidade;
drop table inscricao_mod;
drop table modalidade;
drop table servico;
drop table utente;
Ok, the problem is: the table 'estadia' means 'stay' and 'quarto' means 'room'. I need to, before inserting on 'estadia', to check if that room is available or not.
I may do this:
insert into estadia values (4, 3, *3*, to_date('09.12.15', 'yy.mm.dd'), to_date('10.02.25', 'yy.mm.dd'));
but if, after this command, i do this:
insert into estadia values (5, 4, *3*, to_date('09.12.15', 'yy.mm.dd'), to_date('10.02.25', 'yy.mm.dd'));
it accepts and it shouldn't, because I'm putting users 3 and 4 in the same room at the same time.
the 3rd field is the room id and the next two fields are the entry date (data_entrada) and the exit date (data_saida). If a room is related to a specific stay in which the exit date hasn't gone by, that room shouldn't be available. Also, the exit date may be null, meaning that the exit date is unknown.
I hope to have been clear about my question.
Thanks, Chiapa

Answer 1

This is a fairly easy business rule which can be implemented using triggers. Here goes.
Tables (my version):
drop table stay;
drop table room;
create table room
(room_id     number not null primary key
,other_col     varchar2(10) not null)
create table stay
(stay_id     number not null primary key
,room_id     number not null references room(room_id)
,arrive          date not null check(trunc(arrive)=arrive)
,depart          date          check(trunc(depart)=depart)
,unique (room_id,arrive)
,check(arrive <= depart))
/The rule you are trying to implement is this one:
create or replace assertion no_overlap as
check(not exists
        (select 'two overlapping stays for a room'
         from stay s1
             ,stay s2
         where s1.room_id = s2.room_id
           and s1.stay_id != s2.stay_id
           and (s1.arrive between s2.arrive and nvl(s2.depart,s1.arrive) or
                s1.depart between s2.arrive and nvl(s2.depart,s1.depart))
/Sometime in a far far away future, we may be done now. That future has still to arrive though.
So here's the trigger-code and supporting objects:
create or replace procedure p_get_Lock(p_lockname    in varchar2) as
pl_id number(38);
pl_return number;
pl_timeout number := 0;
begin
  -- Go get a unique lockhandle for this lockname.
  pl_id := dbms_utility.get_hash_value(name      => p_lockname
                                      ,base      => 1
                                      ,hash_size => power(2,30));
  -- Request the application lock in X-mode.
  pl_return :=
  dbms_lock.request(id                => pl_id
                   ,lockmode          => dbms_lock.ssx_mode
                   ,timeout           => pl_timeout
                   ,release_on_commit => true);
  if pl_return not in (0,4)
  then
    raise_application_error(-20000,'Could not serialize for business rule.');
  end if;
end;
create global temporary table stay_te
(room_id     number not null)
create or replace trigger stay_aiur
after insert or update on stay
for each row
begin
  if inserting or
     (updating and (:new.room_id != :old.room_id or
                    :new.arrive < :old.arrive or
                    :new.depart > :old.depart))
  then
    insert into stay_te(room_id) values(:new.room_id);
  end if;
end;
create or replace trigger stay_aius
after insert or update on stay
begin
  for r in (select distinct room_id
            from stay_te)
  loop
    p_get_lock(to_char(r.room_id));
    declare
      p_error varchar2(80);
    begin
      select 'Overlapping stays for room '||to_char(r.room_id)||'.'
      into p_error
      from dual
      where exists
        (select 'two overlapping stays for a room'
         from stay s1
             ,stay s2
         where s1.room_id = s2.room_id
           and s1.room_id = r.room_id
           and s1.stay_id != s2.stay_id
           and (s1.arrive between s2.arrive and nvl(s2.depart,s1.arrive) or
                s1.depart between s2.arrive and nvl(s2.depart,s1.depart))
      raise_application_error(-20000,p_error);
    exception when no_data_found then
      null;
    end;
  end loop;
  delete from stay_te;
end;
/Haven't tested it, but I think it's all okay.