trigger after on insert, error table mutation upon inserting record - sql

Question

I have created a trigger that will update the Payment table (for bill) when a new record is inserted to the table Enrollment. My trigger is as follows:
CREATE OR REPLACE TRIGGER EnrollFee_trig
AFTER INSERT ON Enrollment
FOR EACH ROW
DECLARE
amount Payment.TotalPrice%TYPE;
id Payment.LearnerID%TYPE;
BEGIN
SELECT SUM(Price) into amount
FROM LearnerEnrollCourse_View
WHERE LearnerID = :NEW.LearnerID
AND Paid = 'N';
SELECT LearnerID into id
FROM Payment
WHERE LearnerID = :NEW.LearnerID
AND PaymentDate IS NULL;
IF SQL%FOUND THEN
UPDATE Payment
SET TotalPrice = amount
WHERE LearnerID = :new.LearnerID
AND PaymentDate IS NULL;
ELSE
INSERT INTO Payment VALUES
(PaymentID_Seq.nextval, :new.LearnerID, '', amount);
END IF;
END;
/
The trigger can be created successfully. But when inserting new record into Enrollment table, there is error saying ' table ENROLLMENT is mutating, trigger/function may not see it'. I want to know more specific about what problem causing this and how can I solve it.

Answer 1

Mutating table exceptions occur when we try to reference the triggering table in a query from within row-level trigger code. See more here
In this instance I suspect ( though I don't know as there is no definition for LearnerEnrollCourse_View ) the problem is caused by this statement :-
SELECT SUM(Price) into amount
FROM LearnerEnrollCourse_View
WHERE LearnerID = :NEW.LearnerID
AND Paid = 'N';
If the LearnerEnrollCourse_view view refers to the Enrollment table, you will get the mutating table error. There are a number of ways round it, moving your trigger code into a statement level trigger and holding the data in package variables is one workaround, in general though, I think using triggers is not the best way to do this, the more triggers you have, the more likely you are to run into this and other problems. Instead, I would have an api package for the enrollment table, and move the trigger code into there.
Good discussion of triggers here