SQL Table is mutating... Error - oracle

Question

I'm attempting to write a trigger that will disallow any room in a hospital to have more than 3 services. The table RoomServices has a room number and a service that it has. So the only way to determine this is to group the rooms by room number and count the services. I have tried the code:
CREATE TRIGGER RoomServiceLimit
BEFORE INSERT OR UPDATE ON RoomServices
FOR EACH ROW
DECLARE
numService NUMBER;
CURSOR C1 IS SELECT count(*) AS RoomCount FROM RoomServices WHERE roomNumber = :new.roomNumber;
BEGIN
IF(inserting) THEN
SELECT count(*) into numService FROM RoomServices WHERE roomNumber = :new.roomNumber;
if(numService > 2) THEN
RAISE_APPLICATION_ERROR(-20001,'Room ' || :new.roomNumber || ' will have more than 3 services.');
END IF;
END IF;
IF(updating) THEN
FOR rec IN C1 LOOP
IF(rec.RoomCount > 2) THEN
RAISE_APPLICATION_ERROR(-20001,'Room ' || :new.roomNumber || ' will have more than 3 services.');
END IF;
END LOOP;
END IF;
END;
/
I've tried running each method separately with insert and update, and inserting always works and updating will always give me the mutating table error. I don't know how else to go about solving this problem, so any advice would be greatly appreciated.
Thanks!

Answer 1

There is no reliable way to enforce this kind of constraint using triggers. One possible approach is to use a materialized view that automatically refreshes on commit and has a check constraint enforcing your business rule:
create table roomservices (
pk number not null primary key,
roomnumber number);
create materialized view mv_roomservices
refresh on commit as
select
pk,
roomnumber,
count(*) over (partition by roomnumber) as cnt
from roomservices;
alter table mv_roomservices add constraint
chk_max_2_services_per_room check (cnt <= 2);
Now, whenever you add more than two services for a room and try to commit your transaction, you will get a ORA-12008 exception (error in materialized view refresh path).

Answer 2

I assume that RoomServices:
a) is a small table that is not intensively modified
b) there will never exist a room with more than 3 services
Note: you say "more than 3 services" but your code says "more than 2 services". So I will use "more than 2 services".
Then, what about using a statement trigger?
CREATE OR REPLACE TRIGGER RoomServiceLimit
AFTER INSERT OR UPDATE ON RoomServices
DECLARE
badRoomsCount NUMBER;
badRoomsList VARCHAR2(32767); -- adjust the varchar2 size according to your requirements
BEGIN
SELECT COUNT(*), LISTAGG(roomNumber, ', ') WITHIN GROUP (ORDER BY 1)
INTO badRoomsCount, badRoomsList
FROM (SELECT roomNumber FROM RoomServices GROUP BY roomNumber HAVING COUNT(*) > 2);
IF (badRoomsCount > 0) THEN
RAISE_APPLICATION_ERROR(-20001,'Room/s '||badRoomsList||' will have more than 2 services.');
END IF;
END;
/
If RoomServices is small but have too many changes (inserts or updates) then you may consider create an index on RoomNumber.
If my assumptions are false try something like:
CREATE GLOBAL TEMPORARY TABLE RoomServicesAux as SELECT roomNumber FROM RoomServices WHERE 1=0;
/
CREATE OR REPLACE TRIGGER PreRoomServiceLimit
BEFORE INSERT OR UPDATE ON RoomServices
BEGIN
DELETE FROM RoomServicesAux;
END;
/
CREATE OR REPLACE TRIGGER RowRoomServiceLimit
BEFORE INSERT OR UPDATE OF roomNumber ON RoomServices FOR EACH ROW
BEGIN
INSERT INTO RoomServicesAux VALUES (:NEW.roomNumber);
END;
/
CREATE OR REPLACE TRIGGER RoomServiceLimit
AFTER INSERT OR UPDATE ON RoomServices
DECLARE
badRoomsCount NUMBER;
badRoomsList VARCHAR2(32767); -- adjust the varchar2 size according to your requirements
BEGIN
SELECT COUNT(*), LISTAGG(roomNumber, ', ') WITHIN GROUP (ORDER BY 1)
INTO badRoomsCount, badRoomsList
FROM (
SELECT roomNumber
FROM RoomServices
WHERE roomNumber in (SELECT roomNumber FROM RoomServicesAux)
GROUP BY roomNumber
HAVING COUNT(*) > 2
);
DELETE FROM RoomServicesAux;
IF (badRoomsCount > 0) THEN
RAISE_APPLICATION_ERROR(-20001,'Room/s '||badRoomsList||' will have more than 2 services.');
END IF;
END;
/
Or if you have Oracle 11g or greater then you can use a compound trigger:
CREATE OR REPLACE TYPE RoomsListType IS TABLE OF INTEGER; -- change to the type of RoomServices.rowNumber
/
CREATE OR REPLACE TRIGGER RoomServiceLimit
FOR INSERT OR UPDATE OF roomNumber ON RoomServices
COMPOUND TRIGGER
RoomsList RoomsListType := RoomsListType();
badRoomsCount NUMBER;
badRoomsList VARCHAR2(32767); -- adjust the varchar2 size according to your requirements
AFTER EACH ROW IS
BEGIN
RoomsList.EXTEND;
RoomsList(RoomsList.COUNT) := :NEW.roomNumber;
END AFTER EACH ROW;
AFTER STATEMENT IS
BEGIN
SELECT COUNT(*), LISTAGG(roomNumber, ', ') WITHIN GROUP (ORDER BY 1)
INTO badRoomsCount, badRoomsList
FROM (
SELECT roomNumber
FROM RoomServices
WHERE roomNumber in (SELECT * FROM table(RoomsList))
GROUP BY roomNumber
HAVING COUNT(*) > 2
);
IF (badRoomsCount > 0) THEN
RAISE_APPLICATION_ERROR(-20001,'Room/s '||badRoomsList||' will have more than 2 services.');
END IF;
END AFTER STATEMENT;
END;
/

Answer 3

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;