PL/SQL Triggers Library Infotainment System - sql

Question

I am trying to make a Library Infotainment System using PL/SQL. Before any of you speculate, yes it is a homework assignment but I've tried hard and asking a question here only after trying hard enough.
Basically, I have few tables, two of which are:
Issue(Bookid, borrowerid, issuedate, returndate) and
Borrower(borrowerid, name, status).
The status in Borrower table can be either 'student' or 'faculty'. I have to implement a restriction using trigger, that per student, I can issue only 2 books at any point of time and per faculty, 3 books at any point of time.
I am totally new to PL/SQL. It might be easy, and I have an idea of how to do it. This is the best I could do. Please help me in finding design/compiler errors.
CREATE OR REPLACE TRIGGER trg_maxbooks
AFTER INSERT ON ISSUE
FOR EACH ROW
DECLARE
BORROWERCOUNT INTEGER;
SORF VARCHAR2(20);
BEGIN
SELECT COUNT(*) INTO BORROWERCOUNT
FROM ISSUE
WHERE BORROWER_ID = :NEW.BORROWER_ID;
SELECT STATUS INTO SORF
FROM BORROWER
WHERE BORROWER_ID = :NEW.BORROWER_ID;
IF ((BORROWERCOUNT=2 AND SORF='STUDENT')
OR (BORROWERCOUNT=3 AND SORF='FACULTY')) THEN
ROLLBACK TRANSACTION;
END IF;
END;

Answer 1

Try something like this:
CREATE OR REPLACE TRIGGER TRG_MAXBOOKS
BEFORE INSERT
ON ISSUE
FOR EACH ROW
BEGIN
IF ( :NEW.BORROWERCOUNT > 2
AND :NEW.SORF = 'STUDENT' )
OR ( :NEW.BORROWERCOUNT > 3
AND :NEW.SORF = 'FACULTY' )
THEN
RAISE_APPLICATION_ERROR (
-20001,
'Cannot issue beyond the limit, retry as per the limit' );
END IF;
END;
/
There should not be a commit or rollback inside a trigger. The logical exception is equivalent to ROLLBACK

Answer 2

This is so ugly I can't believe you're being asked to do something like this. Triggers are one of the worst ways to implement business logic. They will often fail utterly when confronted with more than one user. They are also hard to debug because they have hard-to-anticipate side effects.
In your example for instance what happens if two people insert at the same time? (hint: they won't see the each other's modification until they both commit, nice way to generate corrupt data :)
Furthermore, as you are probably aware, you can't reference other rows of a table inside a row level trigger (this will raise a mutating error).
That being said, in your case you could use an extra column in Borrower to record the number of books being borrowed. You'll have to make sure that the trigger correctly updates this value. This will also take care of the multi-user problem since as you know only one session can update a single row at the same time. So only one person could update a borrower's count at the same time.
This should help you with the insert trigger (you'll also need a delete trigger and to be on the safe side an update trigger in case someone updates Issue.borrowerid):
CREATE OR REPLACE TRIGGER issue_borrower_trg
AFTER INSERT ON issue
FOR EACH ROW
DECLARE
l_total_borrowed NUMBER;
l_status borrower.status%type;
BEGIN
SELECT nvl(total_borrowed, 0) + 1, status
INTO l_total_borrowed, l_status
FROM borrower
WHERE borrower_id = :new.borrower_id
FOR UPDATE;
-- business rule
IF l_status = 'student' and l_total_borrowed >= 3
/* OR faculty */ THEN
raise_application_error(-20001, 'limit reached!');
END IF;
UPDATE borrower
SET total_borrowed = l_total_borrowed
WHERE borrower_id = :new.borrower_id;
END;
Update: the above approach won't even work in your case because you record the issue date/return date in the issue table so the number of books borrowed is not a constant over time. In that case I would go with a table-level POST-DML trigger. After each DML verify that every row in the table validates your business rules (it won't scale nicely though, for a solution that scales, see this post by Tom Kyte).