mySQL finding available book and loaning it - mysql

Question

I have created a library system and i'm trying to set up a procedure which allows you to enter an isbn and student number and from that it checks the database to see if the book is available if it is, book it. If not then give an error message.
My tables are as follows:
book - pk_isbn, author, title
copy - pk_code, duration, fk_isbn
loan - fk_code, fk_no, pk_taken, due, return_date
student - pk_no, name, school, allowedtobook
Here is what i currently have code wise *some is just sorta pseudo code i guess as it doesn't work at all currently.
DELIMITER $$
CREATE PROCEDURE issue_loan (IN book_isbn CHAR(17), IN stu_no INT)
BEGIN
DECLARE availablebooks INT;
DECLARE allowedtobook BIT;
SET availablebooks = (SELECT DISTINCT
c.isbn
FROM
loan l
INNER JOIN
student s ON l.no = s.no
INNER JOIN
copy c ON l.code = c.code
WHERE
return_date IS NOT NULL);
IF(availablebooks = book_isbn) AND s.embargo = 0 THEN
INSERT INTO loan VALUES
(code, stu_no, CURDATE(), CURDATE()+c.duration,'');
ELSE
SIGNAL SQLSTATE '450000' SET MESSAGE_TEXT ='no Books Available';
END IF;
END$$
The part which return_date IS NOT NULL should be correct as it finds any books which haven't been returned yet. Meaning they're currently booked out.

Answer 1

You can't set a variable to a query result set, a variable can only hold one value. You should simply add the comparison with book_isbn and s.embargo = 0 to the query.
SET #book_available = (SELECT COUNT(*)
FROM loan l
INNER JOIN student s ON l.no = s.no
INNER JOIN copy c ON l.code = c.code
WHERE c.fk_isbn = book_isbn AND s.embargo = 0 AND l.return_date IS NULL);
IF #book_available
THEN
INSERT INTO loan (fk_code, fk_no, pk_taken, due, return_date)
VALUES (code, stu_no, CURDATE(), CURDATE()+c.duration,'');
ELSE SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT ='no Books Available';
END IF