通过检查其他表中的条件来限制MySQL表中的记录数

Question

I am creating a very simple database system for book rentals. I have a basic MySQL database and a PHP front-end. I'm familiar with html/css/php/js/ajax, but there is one particular piece of functionality that I can't get right.

So I have a books table and a loans table. Each book has a "quantity" attribute, and each loan has a "book name (foreign key)", "issue date", and "return date" attribute.

I'd like to prevent new loans being created for books where every copy is already on loan.

I'm not sure the best way to go about this - is it common practise to put some logic in the database itself, and monitor the number of copies on loan with an extra table or field? Or is it better to handle this kind of logic in the front-end?

Thanks in advance.

Answer 1

There are several possibilities to handle this. Doing it on DB level is a good practice to make sure the data is always correct.

One option is a stored procedure that is used for laoning a book. It would check the tables and insert only if books are available. The downside is that if you don't use the procedure you have no garanties that the data is correct.

Another option would be a trigger that throws an error if one tries to insert into the loan table without any books available. To throw an error and prevent an insert you can use this line of code in your trigger:

SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'insert not allowed because bla bla';

Another option would be using a table that holds a record for every book available and you just set the isAvailable flag in there or dates indicating if it is available or not. The structure could be like this

books table
-----------
id
title
author
...


book_copies table
-----------------
book_id
book_copy_number


loans table
-----------
id
book_copy_id
loan_date
return_date