SQL Table is mutating... Error

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

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;

Answer 3

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;
/