Error when inserting data .. to check the functionality of a trigger .

Question

hallo,
im trying to write a trigger like that :
CREATE OR REPLACE TRIGGER raum_besetzt
BEFORE DELETE OR UPDATE OR INSERT ON BUCHUNGEN
FOR EACH ROW
DECLARE
v_p NUMBER;
v_t NUMBER;
BEGIN
SELECT ANzahl_plaetze into v_p
FROM RAEUME;
SELECT ANzahl_teilnehmer into v_t
FROM BUCHUNGEN;
INSERT INTO BUCHUNGEN VALUES (:NEW.BUCH_ID, :NEW.RAUM_ID, :NEW.MITA_ID, :NEW.VON, :NEW.BIS, :NEW.ANZAHL_TEILNEHMER, :NEW.VART_ID);
IF (v_t > v_p) THEN RAISE_APPLICATION_ERROR (-20001, ' diesen RAUM ist leer ');
END IF;
END;the trigger have to check that all participants will be booked in the room can be accommodated. On error, the reservation is cancel.
so i implemented it the way that number of participants ( anzahl_teilnehmer ) in the Table Reservations ( Buchungen ) is greater than the number of places ( Anzahl_plaetze ) in the table Rooms ( Raeume )
so now i want to add some data to the table BUCHUNGEN ( Reservations ), so i can see if the trigger works!
What i get is the Error below !! :
Fehler beim Start in Zeile 56 in Befehl:
INSERT INTO BUCHUNGEN (BUCH_ID, RAUM_ID, MITA_ID, VON, BIS, ANZAHL_TEILNEHMER, VART_ID)
VALUES                (5, 5, 5, '', '', 30, 3)
Fehlerbericht:
SQL-Fehler: ORA-01422: Exakter Abruf gibt mehr als die angeforderte Zeilenzahl zurück
ORA-06512: in "RAUM_BESETZT", Zeile 5
ORA-04088: Fehler bei der Ausführung von Trigger 'RAUM_BESETZT'
01422. 00000 -  "exact fetch returns more than requested number of rows"
*Cause:    The number specified in exact fetch is less than the rows returned.
*Action:   Rewrite the query or change number of rows requestedcan anyone help :)
thank you ..

Answer 1

Logic such as this should be written in a little api. A trigger is possible, but it might be pretty difficult to implement the full solution in a trigger which has several restrictions. An api usually doesn't have the same technical restrictions (trigger need to prevent endless loops, therefore they will raise an mutating table error if you insert into the same table)
The main issue is that during the execution of your trigger, another process might add a booking without commiting it yet. But your trigger can't see this new booking from the different session. This has to be solved on a logical level first before solving it on database level. How do you want to handle such a situation?