In the DB as in Off documentation, examples of the design of communication Many-to-Many through intermediate tables.

Situation with authors and books. The book can be written by several authors, the author can write a few books.

Below I apply an example code

create table books
(
  Book_id Integer Not Null,
  Book_name Varchar (32) Not NULL,
)
Create Table Autors.
(
  Autor_ID Integer Not Null,
  Autor_Name Varchar (32) Not NULL,
)
Create Table Book_autor
(
  Book_id Integer Not Null,
  Autor_ID Integer Not Null,
  PRIMARY KEY (book_id, autor_id),
  Foreign Key (book_id) References Books,
  Foreign Key (Autor_ID) References Autors
);

The question is, why in the intermediate table PRIMARY KEY (book_id, autor_id). This leads to a ratio of 1-k-1?

Where it would be more logical

create table book_autor
  (
    ID Integer Not Null,
    Book_id Integer Not Null,
    Autor_ID Integer Not Null,
    PRIMARY KEY (ID),
    Foreign Key (book_id) References Books,
    Foreign Key (Autor_ID) References Autors
  );

But there is no such in the documentation. How it all correctly works as an example from the documentation in the links Many-to-Many .

Answer 1, Authority 100%

Absolutely logical structure. It does not lead to one to one. Suppose we have books with ID 1, 2, 3 and authors with ID 100, 101, 102

Pramary Key will not allow insert in the book_Autor table twice recording (1,100). But it will allow you to fill it like this:

book_id autor_id
1 100.
1 102.
2 101.
2 102.
3 100.
3 101.
3 102.

Thus, we have a connection of many-to-many. Book 1 wrote the authors 100 and 102. The author 100 wrote a book 1 and 3.

And the structure with a separate column ID and PRIMARY KEY will allow us to create records:

ID Book_id autor_id
 1 1100
 2 1 100.
 3 1100

That will probably mean that the author wrote the book 1 100 three times. Those. this table structure will not be able to us to ensure the participation of the author in writing a book only once.