Creating an Oracle trigger to stop people enrolling twice - sql

Question

I am facing some problems while I was trying to create a trigger. So the question I was giving is
The event holder has noted that several participants have enrolled in multiple events in the same exhibition which they do not wish to occur. Write a trigger to prevent this issue in future exhibition.
What I have written is
CREATE OR REPLACE TRIGGER check_event_enrolment
BEFORE INSERT ON entry
FOR EACH ROW
DECLARE
new_exhibiton_date DATE;
new_part_no NUMBER;
BEGIN
new_carn_date := (SELECT to_char(carn_date, 'DD/MM/YYYY')
FROM event
WHERE event_id = :new.event_id);
IF (new_carn_date IN (SELECT TO_CHAR(carn_date, 'DD/MM/YYYY')
FROM entry NATURAL
JOIN event
WHERE part_no = :new.part_no)) THEN
raise_application_error(-20001,
'The participant has already enrolled under the same exhibition');
END IF;
END;
Assume that an exhibition can have multiple events going on, the part
new_carn_date := (SELECT to_char(carn_date, 'DD/MM/YYYY')
FROM event
WHERE event_id = :new.event_id);
is giving me error and I dont know how to fix it.
Thank you in advance for your help. If you need more background information I will be more than happy to provide it.
===============================================================
I have tried using one of the solutions provided below, the trigger can be compiled but there's error.
CREATE OR REPLACE TRIGGER check_event_enrolment BEFORE
INSERT ON entry
FOR EACH ROW
DECLARE
new_exhibition_date DATE;
BEGIN
SELECT
exhibition_date
INTO new_exhibition_date
FROM
event
WHERE
event_id = :new.event_id;
IF new_exhibition_date IN (
SELECT
exhibition_date
FROM
entry
NATURAL JOIN event
WHERE
part_no = :new.part_no
) THEN
raise_application_error(-20001, 'The participant has already enroled under the same exhibition');
END IF;
END;
But it is giving me error like
Trigger CHECK_EVENT_ENROLMENT compiled
LINE/COL ERROR
--------- ------------------------------------------------------------- 18/21 PLS-00103: Encountered the symbol "JOIN" when expecting one of the following: ) , with group having intersect minus start union where connect The symbol "," was substituted for "JOIN" to continue. Errors: check compiler log
Anyone knows what is going on?

Answer 1

Don't use a trigger, use a UNIQUE index:
CREATE UNIQUE INDEX entry__carn_dt__part__no__uidx
ON entry (event_id, TRUNC(carn_date), part_no);
If you must use a trigger (don't) then:
CREATE OR REPLACE TRIGGER check_event_enrolment
BEFORE INSERT ON entry
FOR EACH ROW
DECLARE
num_matches PLS_INTEGER;
BEGIN
SELECT COUNT(*)
INTO num_matches
FROM entry
WHERE TRUNC(carn_date) = TRUNC(:NEW.carn_date)
AND event_id = :NEW.event_id
AND part_no = :NEW.part_no;
IF num_matches > 0 THEN
raise_application_error(
-20001,
'The participant has already enrolled under the same exhibition'
) ;
END IF;
END;
/

Answer 2

The syntax you are using is incorrect. The correct syntax sold be -
CREATE OR REPLACE TRIGGER check_event_enrolment BEFORE
INSERT ON entry
FOR EACH ROW
DECLARE
num_matches NUMBER;
BEGIN
SELECT COUNT(*)
INTO num_matches
FROM entry
NATURAL JOIN event
WHERE part_no = :new.part_no
AND event_id = :new.event_id;
IF (num_matches > 0) THEN
raise_application_error (-20001, 'The participant has already enroled under the same exhibition' ) ;
END IF;
END;