Oracle trigger checks previous record before insert/update - oracle

Question

I would like to create a trigger that prevents a student from enrolling into a new module if he has any outstanding bills.
studID studNRIC paymentStatus
-------------------------------------
200 F7654672F Non Payment
it would reject the following statement:
INSERT INTO student(studID, studNRIC, paymentStatus)
VALUES (201, 'F7654672F', 'Good');
I've came out with the following trigger but I'm still able to insert a new student.
set define off;
CREATE OR REPLACE TRIGGER reject_new_account
AFTER INSERT OR UPDATE ON Student
FOR EACH ROW
DECLARE
PRAGMA AUTONOMOUS_TRANSACTION;
totover NUMBER(3);
BEGIN
SELECT COUNT (*)
INTO totover
FROM Student
WHERE :NEW.nric = student.nric
AND :NEW.paymentStatus = 'Non Payment';
IF totover > 0 THEN
RAISE_APPLICATION_ERROR ( -20002,
'Student' || :NEW.nric ||
' has outstanding bills' );
END IF;
END;
/
there's seems to be a problem with line 13 AND :NEW.paymentStatus = 'Non Payment';
so how do I go about doing this?
table structure
CREATE TABLE Student(
studID INTEGER NOT NULL,
firstName CHAR(25) NULL,
lastName CHAR(25) NULL,
NRIC CHAR(9) NOT NULL,
paymentStatus CHAR(25) Default 'Good',
CONSTRAINT stud_Pkey PRIMARY KEY (studID),
CONSTRAINT studPaymentStatus_type CHECK (PaymentStatus IN ('Late Payment', 'Non Payment', 'Good'))
);

Answer 1

There are a couple of issues with this trigger.
First if you want to prevent an INSERT you need to use a BEFORE trigger. An AFTER trigger will fire after the insert has successfully completed, by which point it is too late to stop the insert.
Secondly, I'm unsure about what you are trying to achieve with your SQL statement. Since the trigger is attached to the customer table you don't need to do a select on the customer table to access the record data. You can just do:
IF :NEW.badstatus = 'Non Payment'
THEN
RAISE_APPLICATION_ERROR ( -20002,
'Employee ' || :NEW.nric ||
' already has 5 readings' );
END IF;

Answer 2

You probably don't want to check if the new record has a bad status, you want to check if the existing customer records have a bad status. As James mentioned, a BEFORE trigger probably makes more sense, but that depends on what you are trying to do, whether the insert gets rollback or not by the caller, etc.
CREATE OR REPLACE TRIGGER reject_new_account
AFTER INSERT OR UPDATE ON Customer
FOR EACH ROW
DECLARE
PRAGMA AUTONOMOUS_TRANSACTION;
totover NUMBER(3);
BEGIN
SELECT COUNT (*)
INTO totover
FROM Customer c
WHERE c.nric = :NEW.nric
AND c.badstatus = 'Non Payment';
IF totover > 0 THEN
RAISE_APPLICATION_ERROR ( -20002,
'Employee ' || :NEW.nric ||
' already has 5 readings' );
END IF;
END;
/

Answer 3

I've removed the PRAGMA AUTONOMOUS_TRANSACTION; for you and replace it with an exception to handle the issue, can you try if it works.
set define off;
CREATE OR REPLACE TRIGGER reject_new_account
BEFORE INSERT OR UPDATE ON CUSTOMER
FOR EACH ROW
DECLARE
totover CHAR(100);
BEGIN
SELECT distinct badStatus
INTO totover
FROM customer
WHERE :NEW.nric = CUSTOMER.nric;
IF totover = 'Non Payment' THEN
RAISE_APPLICATION_ERROR ( -20003,
'Customer ' || :NEW.firstName||''||:NEW.lastName ||
' cannot create new account due to bad payment' );
END IF;
EXCEPTION
WHEN NO_DATA_FOUND
THEN DBMS_OUTPUT.PUT_LINE(TO_CHAR(SQLERRM(-20299)));
END;
/

Answer 4

As I assume you have discovered, you cannot select from the same table that a row-level trigger is defined against; it causes a table mutating exception. You have attempted to get round this by adding the autonomous transaction pragma. Unfortunately, although this works, it is just covering up your mistake in methodology.
In order to properly create this validation using a trigger a procedure should be created to obtain user-specified locks so the validation can be correctly serialized in a multi-user environment.
PROCEDURE request_lock
(p_lockname IN VARCHAR2
,p_lockmode IN INTEGER DEFAULT dbms_lock.x_mode
,p_timeout IN INTEGER DEFAULT 60
,p_release_on_commit IN BOOLEAN DEFAULT TRUE
,p_expiration_secs IN INTEGER DEFAULT 600)
IS
-- dbms_lock.allocate_unique issues implicit commit, so place in its own
-- transaction so it does not affect the caller
PRAGMA AUTONOMOUS_TRANSACTION;
l_lockhandle VARCHAR2(128);
l_return NUMBER;
BEGIN
dbms_lock.allocate_unique
(lockname => p_lockname
,lockhandle => p_lockhandle
,expiration_secs => p_expiration_secs);
l_return := dbms_lock.request
(lockhandle => l_lockhandle
,lockmode => p_lockmode
,timeout => p_timeout
,release_on_commit => p_release_on_commit);
IF (l_return = 1) THEN
raise_application_error(-20001, 'dbms_lock.request Timeout');
ELSIF (l_return = 2) THEN
raise_application_error(-20001, 'dbms_lock.request Deadlock');
ELSIF (l_return = 3) THEN
raise_application_error(-20001, 'dbms_lock.request Parameter Error');
ELSIF (l_return = 5) THEN
raise_application_error(-20001, 'dbms_lock.request Illegal Lock Handle');
ELSIF (l_return not in (0,4)) THEN
raise_application_error(-20001, 'dbms_lock.request Unknown Return Value ' || l_return);
END IF;
-- Must COMMIT an autonomous transaction
COMMIT;
END request_lock;
This procedure can then be used in a compound trigger (assuming at least Oracle 11, this will need to be split into individual triggers in earlier versions)
CREATE OR REPLACE TRIGGER reject_new_account
FOR INSERT OR UPDATE ON student
COMPOUND TRIGGER
-- Table to hold identifiers of inserted/updated students
g_studIDs sys.odcinumberlist;
BEFORE STATEMENT
IS
BEGIN
-- Reset the internal student table
g_studIDs := g_studIDs();
END BEFORE STATEMENT;
AFTER EACH ROW
IS
BEGIN
-- Store the inserted/updated students; the payment status may be updated
-- without checking the constraint
IF ( INSERTING
OR ( UPDATING
AND ( :new.studID <> :old.studID
OR :new.NRIC <> :old.NRIC)))
THEN
g_studIDs.EXTEND;
g_studIDs(g_studIDs.LAST) := :new.studID;
END IF;
END AFTER EACH ROW;
AFTER STATEMENT
IS
CURSOR csr_students
IS
SELECT sdt.studID
, sdt.NRIC
FROM TABLE(g_studIDs) sid
INNER JOIN student sdt
ON (sdt.studID = sid.column_value)
ORDER BY sdt.NRIC;
CURSOR csr_constraint_violations
(p_studID student.studID%TYPE
,p_NRIC student.studNRIC%TYPE)
IS
SELECT NULL
FROM student sdt
WHERE sdt.NRIC = p_NRIC
AND sdt.paymentStatus = 'Bad Payment'
AND sdt.studID <> p_studID;
r_constraint_violation csr_constraint_violations%ROWTYPE;
BEGIN
-- Check if for any inserted/updated student there exists another record for
-- the same NRIC with a Bad Payment status. Serialise the constraint for each
-- NRIC so concurrent transactions do not affect each other
FOR r_student IN csr_students LOOP
request_lock('REJECT_NEW_ACCOUNT_' || r_student.NRIC);
OPEN csr_constraint_violations(r_student.studID, r_student.NRIC);
FETCH csr_constraint_violations INTO r_constraint_violation;
IF csr_constraint_violations%FOUND THEN
CLOSE csr_constraint_violations;
raise_application_error(-20001, 'Student ' || r_student.NRIC || ' has Bad Payment status');
ELSE
CLOSE csr_constraint_violations;
END IF;
END LOOP;
END AFTER STATEMENT;
END;