Restrict date range to be entered

Question

G'day
I have a table called guest that allows them to be "checked in" in a hotel for example.
The table was built as:
CREATE TABLE GUEST
   (     ROOM_NUMBER VARCHAR2(2),
     DATE_ARRIVAL DATE NOT NULL,
     DATE_DEPARTURE DATE NOT NULL,
     NUM_ADULTS INTEGER,
     NUM_CHILDREN INTEGER,
     GUEST_NAME VARCHAR2(60),
     ADDRESS VARCHAR2(40),
     LICENSE_PLATE VARCHAR2(10),
           CONSTRAINT LICENSEPLATE_UN UNIQUE (LICENSE_PLATE),
           CONSTRAINT GUEST_ARRIVALDATE_CHK CHECK (DATE_ARRIVAL <= DATE_DEPARTURE), 
     CONSTRAINT GUEST_ROOM_FK FOREIGN KEY (ROOM_NUMBER)
     REFERENCES ROOM (ROOM_NUMBER));I'd like to add another check constraint to ensure that if a room has alredy been allocated to another guest :date arrival (10-May-06) to date departure (15-May-06) cannot be given to another guest within that date range.
Some dumb data to be tested
INSERT INTO Guest VALUES('R1','10-MAY-06','15-MAY-06',1,1,'Tom','London','AH343');
INSERT INTO Guest VALUES('R1','14-MAY-06','17-MAY-06',2,0,'John','Canada','AF343');Thanx

Answer 1

There was rather elegant solution discussed on AskTom. I do not remenber details, but it goes something like this:
CRETATE MATERIALIZED VIEW MV AS
SELECT G1.ROOM_NUMBER
,G1.DATE_ARRIVAL
,G1.DATE_DEPARTURE
,G2.DATE_ARRIVAL DATE
,G2.DATE_DEPARTURE
FROM GUEST G1, GUEST G2
WHERE G1.ROOM_NUMBER = G2.ROOM_NUMBER
AND G1.DATE_ARRIVAL < G2.DATE_DEPARTURE
AND G2.DATE_ARRIVAL < G1.DATE_DEPARTURE
REFRESH ON COMMIT
And then set some kind of restriction on the view that prohibits it from having any rows.
BTW trigger proposal would not work because of "table is mutating" error.
Checks on app level would require row locks (SELECT FOR UPDATE), otherwise sooner or later you will end up with several guys in the same room:)