PL/SQL Triggers - sql

Question

I am trying to make a Library Information System. I have a table called Borrower(borrower_id: number, name: varchar2(30), status: varchar2(20)). 'status' can be either 'student' or 'faculty'.
I have a restriction that a maximum of 2 books can be issued to a student at any point of time, and 3 to a faculty. How do I implement it using triggers?
This is a homework question. But I've tried hard to come up with some logic. I am new to SQL so this might be easy for you lot but not for me.
I am new to stackexchange, so sorry if I've violated some rules/practices.

Answer 1

I expect that you would maintain a count on the borrower table of the number of books borrowed, and modify it via a trigger when a book is borrowed and when it is returned. Presumably you also have a table for the books being borrowed by the user, and the trigger would be placed on that take.
A constraint on the books_borrowed column could raise an error if the count of borrowed books exceeds 2.

Answer 2

This is a pretty old question, but I found it very useful as I'm also a PL/SQL beginner. There are two approaches to solve the problem and the one you want to use depends on the Oracle DB version.
For older version use a combination of a trigger and a package as below.
CREATE OR REPLACE TRIGGER trg_borrower
BEFORE INSERT OR UPDATE ON borrower
FOR EACH ROW
DECLARE
v_count NUMBER := 0;
BEGIN
v_count := borrower_pkg.count_rows(:NEW.borrower_id, :NEW.name, :NEW.status);
IF :NEW.status = 'student' AND v_count = 2 THEN
RAISE_APPLICATION_ERROR(-20000, 'Error - student');
ELSIF :NEW.status = 'faculty' AND v_count = 3 THEN
RAISE_APPLICATION_ERROR(-20001, 'Error - faculty');
END IF;
END;
/
CREATE OR REPLACE PACKAGE borrower_pkg AS
FUNCTION count_rows(p_id IN borrower.borrower_id%TYPE,
p_name IN borrower.NAME%TYPE,
p_status IN borrower.status%TYPE) RETURN NUMBER;
END;
/
CREATE OR REPLACE PACKAGE BODY borrower_pkg AS
FUNCTION count_rows(p_id IN borrower.borrower_id%TYPE,
p_name IN borrower.NAME%TYPE,
p_status IN borrower.status%TYPE) RETURN NUMBER AS
v_count NUMBER := 0;
BEGIN
SELECT COUNT(*) INTO v_count
FROM borrower
WHERE borrower_id = p_id AND NAME = p_name AND status = p_status;
RETURN v_count;
END count_rows;
END borrower_pkg;
/
For Oracle 10g and above you can use a compound trigger.
CREATE OR REPLACE TRIGGER trg_borrower_comp
FOR INSERT OR UPDATE ON borrower
COMPOUND TRIGGER
CURSOR c_borrower IS
SELECT b1.borrower_id
FROM borrower b1
WHERE EXISTS (SELECT 'x'
FROM borrower b2
WHERE b2.status = 'student' AND b1.borrower_id = b2.borrower_id
GROUP BY borrower_id HAVING COUNT(*) = 2)
OR
EXISTS (SELECT 'x'
FROM borrower b3
WHERE status = 'faculty'AND b1.borrower_id = b3.borrower_id
GROUP BY borrower_id HAVING COUNT(*) = 3);
TYPE t_borrower_count IS TABLE OF borrower.borrower_id%type;
v_borrower_count t_borrower_count;
BEFORE STATEMENT IS
BEGIN
OPEN c_borrower;
FETCH c_borrower BULK COLLECT INTO v_borrower_count;
CLOSE c_borrower;
END BEFORE STATEMENT;
BEFORE EACH ROW IS
BEGIN
IF :NEW.borrower_id MEMBER OF v_borrower_count THEN
RAISE_APPLICATION_ERROR(-20000, 'Error - ' || :NEW.status);
END IF;
END BEFORE EACH ROW;
END;