Select 行，以便其中两列分别唯一

Question

Table user_book describes every user's favorite books.

CREATE TABLE user_book (
  user_id INT,
  book_id INT,
  FOREIGN KEY (user_id) REFERENCES user(id),
  FOREIGN KEY (book_id) REFERENCES book(id)
);

insert into user_book (user_id, book_id) values
(1, 1),
(1, 2),
(1, 5),
(2, 2),
(2, 5),
(3, 2),
(3, 5);

I want to write a query (possibly a with clause that defines multiple statements ― but not a procedure) that would try to distribute ONE favorite book to every user who has one or more favorite books.

Any ideas how to do it?

More details:

The distribution plan may be naive. ie it may look as if you went user after user and each time randomly gave the user whatever favorite book was still available if there was any, without considering what would be left for the remaining users.

This means that sometimes some books may not be distributed, and/or sometimes some users may not get any book (example 2). This can happen when the numbers of books and users are not equal, and/or due to the specific distribution order that you have used.

A book cannot be distributed to two different users (example 3).

Examples:

1. A possible distribution:

(1, 1)
(2, 2)
(3, 5)

2. A possible distribution (here user 3 got nothing, and book 1 was not distributed. That's acceptable):

(1, 2)
(2, 5)

3. An impossible distribution (both users 1 and 2 got book 2, that's not allowed):

(1, 2)
(2, 2)
(3, 5)

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Answer 1

The user_book table should also have a UNIQUE(user_id, book_id) constraint.

A simple solution like this returns a list in which each user gets zero or one book and each book is given to zero or one user:

WITH list AS (SELECT user_id, MIN(book_id) AS fav_book FROM user_book GROUP BY user_id)
SELECT fav_book, MIN(user_id) FROM list GROUP BY fav_book