MySQL inner join on two columns - php

Question

I have two tables, books and authors. books has a author_id column and a secondary_author_id column (no books have more than two authors). I'm so far doing:
SELECT * FROM books
LEFT JOIN authors
ON books.author_id=authors.id
which is handling the join with the first author. I can't work out how I'd handle the secondary author though. Should I change my schema, or do I just need a bit of SQL help?

Answer 1

SELECT books.*, author1.*, author2.*
FROM books
LEFT JOIN author AS author1
ON author1.author_id = books.author_id
LEFT JOIN author AS author2
ON author2.author_id = books.secondary_author_id
In SQL, you can alias the tables by adding it after the table name. Just be careful, now you'll have duplicate columns, so instead of author1.* you will probably want to alias the results of both author1 and author2.
EDIT
Additional details -- Say you have your basic table (i'll include the details so if people want to test on their own they can):
CREATE DATABASE test;
USE test;
CREATE TABLE books
(
book_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
title VARCHAR(50),
author_id INT NOT NULL,
secondary_author_id INT
);
CREATE TABLE authors
(
author_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(50)
);
INSERT INTO authors (author_id,name) VALUES (1,'Sue Z. Que'),(2,'John Doe'),(3,'Bob Smith');
INSERT INTO books (book_id,title,author_id,secondary_author_id) VALUES (1,'JOIN-ing Two Tables',1,2);
If you do the select I mention above, your result will be the following:
|----------------------- books TABLE -----------------------------|---- authors table -----|---- authors table ---|
+---------+---------------------+-----------+---------------------+-----------+------------+-----------+----------+
| book_id | title | author_id | secondary_author_id | author_id | name | author_id | name |
+---------+---------------------+-----------+---------------------+-----------+------------+-----------+----------+
| 1 | JOIN-ing Two Tables | 1 | 2 | 1 | Sue Z. Que | 2 | John Doe |
+---------+---------------------+-----------+---------------------+-----------+------------+-----------+----------+
(I've added the top header just for calrity's sake) you see you have two author_id's and two name's (as they are joins of the same table and same column names). BUT, if you alias the columns from the joins like so:
SELECT books.*, author1.name AS primary_author, author2.name AS secondary_author
FROM books
LEFT JOIN authors AS author1
ON author1.author_id = books.author_id
LEFT JOIN authors AS author2
ON author2.author_id = books.secondary_author_id;
You get a much cleaner result:
|----------------------- books TABLE -----------------------------| authors table -|- authors table --|
+---------+---------------------+-----------+---------------------+----------------+------------------+
| book_id | title | author_id | secondary_author_id | primary_author | secondary_author |
+---------+---------------------+-----------+---------------------+----------------+------------------+
| 1 | JOIN-ing Two Tables | 1 | 2 | Sue Z. Que | John Doe |
+---------+---------------------+-----------+---------------------+----------------+------------------+

Answer 2

SELECT books.* FROM books, authors.name, secondary_authors.name
LEFT JOIN authors
ON books.author_id=authors.id
LEFT JOIN authors as secondary_authors
ON books.secondary_author_id=secondary_authors.id

Answer 3

You need to rethink your design, because one day there will be a book with three authors, and the next day there will be a book with zero. (I've been there myself.)
Edit
As your comment says: yes, you need a books_authors table. As long as you have your indexes set up properly, it's not a big performance hit.
The most annoying part is that you're often going to want to string the authors together (one entry per book, concatenating all the authors into a single column). You'll probably end up creating a view for that.

Answer 4

just do another join on the secondary id