Seems you cannot solve this issue without some workarounds. If there is nothing better you can find, check this out:
I guess you have table Room, otherwise create one:
alter table Room add (
servicesCount integer default 0 not null check (servicesCount <= 3)
);
Then update this number with current values (not sure if the statement is valid, it is not the key point here)
update Room r
set servicesCount = (select count(*)
from RoomServices s
where r.roomNumber = s.roomNumber);
then in your trigger
create trigger RoomServiceLimit
before insert or update on RoomServices
for each row
begin
update Room
set servicesCount = servicesCount + 1
where roomNumber = :new.roomNumber;
end;
Looks quite ugly, but, as I've told, I am not sure you can find anything better with trigger.
EDIT
The complete working example
drop table Room;
drop table RoomServices;
create table Room (
roomNumber integer primary key,
servicesCount integer default 0 not null check (servicesCount <= 3)
);
create table RoomServices (
roomNumber integer,
service varchar2(100),
comments varchar2(4000)
);
create trigger RoomServiceLimit
before insert or update or delete on RoomServices
for each row
begin
if inserting then
update Room
set servicesCount = servicesCount + 1
where roomNumber = :new.roomNumber;
elsif updating and :old.roomNumber != :new.roomNumber then
update Room
set servicesCount = servicesCount + 1
where roomNumber = :new.roomNumber;
update Room
set servicesCount = servicesCount - 1
where roomNumber = :old.roomNumber;
elsif deleting then
update Room
set servicesCount = servicesCount - 1
where roomNumber = :old.roomNumber;
end if;
end;
/
insert into Room(roomNumber) values (1);
insert into Room(roomNumber) values (2);
insert into RoomServices(roomNumber,service,comments) values (1,'cleaning','first');
insert into RoomServices(roomNumber,service,comments) values (1,'drying','second');
insert into RoomServices(roomNumber,service,comments) values (1,'watering','third');
insert into RoomServices(roomNumber,service,comments) values (1,'something','third'); -- error
select * from room;
insert into RoomServices(roomNumber,service,comments) values (2,'something','2: first');
update RoomServices
set comments = null
where roomNumber = 2;
select * from room;
update RoomServices -- error
set roomNumber = 1
where roomNumber = 2;
select * from room;
delete from RoomServices where roomNumber = 1;
select * from room;