Error during execution of trigger. How to rework select statement? - oracle-sqldeveloper

Question

I'm relatively new to triggers so forgive me if this doesn't look how it should. I am creating a trigger that checks a user account for last payment date and sets a value to 0 if they haven't paid in a while. I created what I thought was a correct trigger but I get the error, "error during execution of trigger" when its triggered. From what I understand the select statement is causing the error as it selecting values which are in the process of being changed. Here is my code.
CREATE OR REPLACE TRIGGER t
BEFORE
UPDATE OF LASTLOGINDATE
ON USERS
FOR EACH ROW
DECLARE
USER_CHECK NUMBER;
PAYMENTDATE_CHECK DATE;
ISACTIVE_CHECK CHAR(1);
BEGIN
SELECT U.USERID, U.ISACTIVE, UP.PAYMENTDATE
INTO USER_CHECK, PAYMENTDATE_CHECK, ISACTIVE_CHECK
FROM USERS U JOIN USERPAYMENTS UP ON U.USERID = UP.USERID
WHERE UP.PAYMENTDATE < TRUNC(SYSDATE-60);
IF ISACTIVE_CHECK = 1 THEN
UPDATE USERS U
SET ISACTIVE = 0
WHERE U.USERID = USER_CHECK;
INSERT INTO DEACTIVATEDUSERS
VALUES(USER_CHECK,SYSDATE);
END IF;
END;
From what I thought, since the select is in the begin statement, it would run before an update, nothing would be changing about the tables until after if runs through the trigger. I tried but using :old in front of the select variables but that doesn't seem to be the right use.
And here is the update statement i was trying.
UPDATE USERS
SET LASTLOGINDATE = SYSDATE
WHERE USERID = 5;

Answer 1

Some issues:
The select you do in the trigger sets the variable isactive_check to a payment date, and vice versa. There is an accidental switch there, which will have a negative effect on the next if;
The same select should return exactly one record, which by the looks of it is not guaranteed, since you join with table userpayments, which may have several payments for the selected user that meet the condition, or none at all. Change that select to do an aggregation.
If a user has more than one payment record, the condition might be true for one, but not for another. So if you are interested only in users who have not paid in a long while, such user should not be included, even though they have an old payment record. Instead you should check whether all records meet the condition. This you can do with a having clause.
As the table users is mutating (the update trigger is on that table), you cannot perform every action on that same table, as it would otherwise lead to a kind of deadlock. This means you need to rethink what the purpose is of the trigger. As this is about an update for a specific user, you actually don't need to check the whole table, but only the record that is being changed. For that you can use the special new variable.
I would suggest this SQL instead:
SELECT MAX(UP.PAYMENTDATE)
INTO PAYMENTDATE_CHECK
FROM USERPAYMENTS
WHERE USERID = :NEW.USERID
and then continue with the checks:
IF :NEW.ISACTIVE = 1 AND PAYMENTDATE_CHECK < TRUNC(SYSDATE-60) THEN
:NEW.ISACTIVE := 0;
INSERT INTO DEACTIVATEDUSERS (USER_ID, DEACTIVATION_DATE)
VALUES(USER_CHECK,SYSDATE);
END IF;
Now you have avoided to do anything in the table users and have made the checks and modification via the :new "record".
Also, it is good practice to mention the column names in an insert statement, which I have done in above code (adapt column names as needed):
Make sure the trigger is compiled and produces no compilation errors.