将am：n MySQL关系与自己列中的每个实体连接

Question

I'm working on a web application to manage my books and I'm currently confronted with a problem I can't solve.

Setup

• Each book has several information which are stored in the table books . The primary key is bookId and each book can be written by 1 or more authors.
• Each author has a name which together with the primary key authorId is stored in the table authors . Every author can have written 1 or more books.
• Therefore I created a linking table books_authors where the bookId and the authorId are stored as foreign keys to bookId in books and authorId in authors .

See here a shortened version of the relevant tables:

books
+--------+-------+-------------+
| bookId | title | numberPages |
+--------+-------+-------------+
| 1      | Alpha | 100         |
| 2      | Beta  | 200         |
| 3      | Ceta  | 150         |
+--------+-------+-------------+

authors
+----------+----------+
| authorId | name     |
+----------+----------+
| 1        | John     | 
| 2        | Max      |   
| 3        | Gina     |
| 4        | Marry    |
+----------+----------+

books_authors
+--------+----------+
| bookId | authorId |
+--------+----------+
| 1      | 1        |
| 1      | 4        |
| 2      | 2        |
| 3      | 2        |
| 3      | 3        |
| 3      | 4        |
+--------+----------+

Problem

I need a SQL statement now that gives me back one single table like the following. Every book should only have one row and all authors should be in seperate columns. And exactly this is the problem for me in finding a solution as sometimes a book has one or two or three or ... authors. So the statement should be "dynamic" in creating this "joined" table:

Joined Table
+--------+-------+-------------+---------+---------+---------+
| bookId | title | numberPages | author1 | author2 | author3 |
+--------+-------+-------------+---------+---------+---------+
| 1      | Alpha | 100         | John    | Marry   |         |
| 2      | Beta  | 200         | Max     |         |         |
| 3      | Ceta  | 150         | Max     | Gina    | Marry   |
+--------+-------+-------------+---------+---------+---------+

Thank you very much for your help!

Answer 1

I don't like your current expected output, because in the future we may encounter a book which has more than 3 authors. I suggest the following alternative:

SELECT
    b.bookId,
    b.title,
    b.numberPages,
    GROUP_CONCAT(a.name ORDER BY a.authorId) authors
FROM books b
LEFT JOIN books_authors ba
    ON b.bookId = ba.bookId
LEFT JOIN authors a
    ON ba.authorId = a.authorId
GROUP BY
    b.bookId;

This approach generates a CSV list of all authors of each book.