Why doesn't GROUP_CONCAT work?

Question

MySQL

There is a query that selects books and authors for them. There are 3 tables: books, authors, authors_books
It is necessary that all authors associated with it be displayed next to the book.

$sql = "SELECT b.*,a.name,a.surname, GROUP_CONCAT(a.surname) as surname FROM books b"
                . " INNER JOIN authors_books ab on b.id = ab.book_id"
                . " LEFT  JOIN authors a on ab.author_id = a.id";

But with such a request, absolutely all the author is displayed

Answer 1

We look at examples of group_concat in any description and pay attention to the presence of group by there