MYSQL query with 3 INNER JOINs - php

Question

I'm trying to create a book-catalogue. I have 3 basic tables - books, authors, books_authors;
books
+------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+----------------+
| book_id | int(11) | NO | PRI | NULL | auto_increment |
| book_title | varchar(250) | NO | | NULL | |
+------------+--------------+------+-----+---------+----------------+
authors
+-------------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------------+--------------+------+-----+---------+----------------+
| author_id | int(11) | NO | PRI | NULL | auto_increment |
| author_name | varchar(250) | NO | | NULL | |
+-------------+--------------+------+-----+---------+----------------+
books_authors
+-----------+---------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+-----------+---------+------+-----+---------+-------+
| book_id | int(11) | NO | MUL | NULL | |
| author_id | int(11) | NO | MUL | NULL | |
+-----------+---------+------+-----+---------+-------+
I have a query that takes the book name and all authors for each book and displays the result:
$booksAndAuthors = mysqli_query($connection, 'SELECT * FROM books LEFT JOIN books_authors ON books.book_id=books_authors.book_id LEFT JOIN authors ON authors.author_id=books_authors.author_id');
It returns:
Book Name -> Author 1, Author 2
Book Name 2 -> Author 3, Author 2
And so on.
And I have another query that it's:
$booksAndAuthors = mysqli_query($connection, 'SELECT * FROM books_authors as ba
INNER JOIN books as b ON ba.book_id=b.book_id
INNER JOIN books_authors as booaut ON booaut.book_id=ba.book_id
INNER JOIN authors as a ON booaut.author_id=a.author_id
WHERE ba.author_id=' . $author_id);
When I click over an author (authors are links), the query returns all books of an author the opposite; The queries all work;
My Question is:
Could someone explain to me why I'm comparing a table with itself. Just explain for dummie like myself. I want to understand the action that is done by this query, with words or something else.
*If my question isn't properly asked! Edit me!
*Regards!

Answer 1

A book can have more than one author. The point of the self-join is to find the other authors for the book.
FROM books_authors as ba
...
INNER JOIN books_authors as booaut ON booaut.book_id=ba.book_id
...
WHERE ba.author_id=42
The join picks up any author who co-authored a book with author 42.
Another way to write the query:
FROM books_authors as ba
...
WHERE EXISTS
(
SELECT *
FROM books_authors ba2
WHERE ba2.book_id = ba.book_id
and ba2.author_id = 42
)
This says, select all rows where a matching book_authors entry exists for author 42.

Answer 2

It seems a self join ( joining the table with itself ) is unnecessary here since you are picking the same rows.
Usually self joins are performed to join two different rows in a table. For example, if you have a table with monthly account balances for example
acount_id |as_of_date | balance_amount
-----------|---------------------------
12213 |2014-01-01 | 10000
12213 |2014-02-01 | 20000
12213 |2014-03-01 | 25000
Let's say the table name is monthly_account_balances
Now you want to compute the difference between monthly balances
For instance, between February and January the difference is 20000 - 10000 = 10000
And between March and February the difference is 25000 - 20000 = 5000
And the output you need is
acount_id |as_of_date | balance_amount|difference
-----------|-----------|---------------|-------------
12213 |2014-01-01 | 10000 | null
12213 |2014-02-01 | 20000 | 10000
12213 |2014-03-01 | 25000 | 5000
Here you do a self join as follows:
select a.*, b.balance_amount - a.balance_amount as difference
from monthly_account_balances a
inner join monthly_account_balances b on a.account_id = b.account_id
and a.as_of_date + interval '1 month' = b.as_of_date
Notice the date condition. It's comparing two different records with same id but different as_of_date. Self join is useful in such situations.
However in your case you are just joining on id and I see no point in doing that unless I am missing something