Postgresql: One table dependent on another - database

Question

I have three tables in my Database - Students, Books and Books2student.
Students table has StudentID, First Name, Last Name.
Books table has ISBN no, name, no. of copies available.
I want to create a third table Books2Students which has StudentID, books issued, issue date, due date.
How can I create the third table such that if I insert a record in it the no. of copies in Books table should decrease.
If I delete a Student from student table and that student has taken a book then no. of copies in Books table should increased.

Answer 1

You're looking at this the wrong way. The Books table should only have the total amount of copies, not the amount that you need right now. When you query the actual amount of books available, you subtract the amount which has been lent out. You might find that for performance reasons, you want to persist a computed value in the book table itself, but performance optimizations like this come with their own costs :)
Now, whether you do this or update the column manually, you'll need to make sure this works in a concurrency-safe way (so that you don't lend more books than you have, for example). Which gets you into the "distributed systems are hard" territory :)

Answer 2

You can use triggers. It's too simple.
Refer to tutorials: http://www.postgresqltutorial.com/creating-first-trigger-postgresql/
Ignoring the performance part (which is important), here is an example that you should adapt to your table's columns:
---------Creating function-----------
CREATE OR REPLACE FUNCTION student_to_book(p_ID)
RETURNS trigger AS
$BODY$
BEGIN
IF (TG_OP = 'INSERT') THEN
UPDATE Books2Students BS
SET BS.noCopies= noCopies+1
WHERE BS.BOOKS_ISSUED=p_ID;
RETURN NEW;
ELSIF (TG_OP = 'DELETE') THEN
UPDATE Books2Students BS
SET BS.noCopies= noCopies-1
WHERE BS.BOOKS_ISSUED=p_ID;
RETURN OLD;
END IF;
RETURN NULL;
END;
$BODY$
---------Creating TRIGGER-----------
CREATE TRIGGER student_manage
BEFORE INSERT OR DELETE
ON STUDENT
FOR EACH ROW
EXECUTE PROCEDURE student_to_book(ROW.StudentID);