Show earliest reservations where books are available - mysql

Question

I have three tables: loans, reservations and books. The books table has an attribute for "noOfCopies", and the total number of loans and reservations for that book can not exceed that figure.
The reservations table has a "timestamp" column which is just the timestamp that the reservation was made. The idea is that when a book is returned, the earliest reservation gets the book next.
Here is what I need help with: I need to create an SQL view that will show all the earliest reservations for each book, but only where that book is available.
Can anyone give me the SQL for this? Thanks in advance.
Here is the SQL I already had: I thought it showed all the reservations where the books were available and was about to move on to figuring out how to show the earliest - but then I got nowhere with earliest and then realised that this doesn't actually work anyway:
CREATE VIEW `view_bookLoans` AS
SELECT count(l.`id`) as loanCount, l.`bookISBN`,b.`noOfCopies` FROM
loans l INNER JOIN books b
ON l.`bookISBN` = b.`ISBN`
GROUP BY l.`bookISBN`;
CREATE VIEW `view_reservationList` AS
SELECT
r.`timestamp`,
b.`title` as `bookTitle`,
r.`readerID`,
bl.`loanCount`
FROM
`reservations` r INNER JOIN `books` b
ON r.`bookISBN` = b.`ISBN`
LEFT JOIN
view_bookLoans bl
ON bl.`bookISBN` = b.`ISBN`
WHERE
(b.`noOfCopies` - bl.`loanCount`) > 0;

Answer 1

With exception to the comments I put in the question about how to exclude reservations already filled, or loans already returned... This SHOULD do it for you.
I would start the list by only looking at those books that people have reserved. Why query and entire list of books where nobody is interested in them... THEN, expand your criteria. The inner query starts directly with the books on the reservations list. That is LEFT JOINED to the loans table (in case there are none remaining on loan). I'm extracting the ealiest time stamp for the reservation per book, and also getting the total count of LOAN book entries grouped by ISBN.
From that result, I immediately re-join to the reservations to match based on the ISBN and timestamp of the earliest to get the WHO wanted it.
Now the finale... JOIN (not a LEFT JOIN) to the books table on both the ISBN AND your qualifier that the number of copies available... less how many are already loaned out is > 0.
You can obviously add an order by clause however you want.
ENSURE that you have an index on the reservations table on (bookISBN, timestamp) for query optimization. Also, the loans table too should have an index on ISBN.
SELECT
B.Title,
R2.readerID,
R2.timeStamp,
Wanted.AlreadyLoandedOut,
B.noOfCopies - Wanted.AlreadyLoanedOut as ShouldBeAvailable
FROM
( select
R.bookISBN,
MIN( R.timeStamp ) EarliestReservation,
COALESCE( COUNT( L.ID ), 0 ) as AlreadyLoanedOut
from
reservations R
LEFT JOIN Loans L
ON R.bookISBN = L.bookISBN
group by
R.bookISBN ) as Wanted
JOIN reservations R2
ON Wanted.bookISBN = R2.bookISBN
AND Wanted.EarliestReservation = R2.timeStamp
JOIN books B
ON Wanted.bookISBN = B.ISBN
AND B.noOfCopies - Wanted.AlreadyLoanedOut > 0