MySQL: Get all combinations

Question

given I have a user table users:

user_id	name
1	Bob
2	Adam
3	Charlie

Now I want to get all combinations of one or two users:

user1	user2
Bob	NULL
Bob	Adam
Bob	Charlie
Adam	NULL
Adam	Bob
Adam	Charlie
Charlie	NULL
Charlie	Bob
Charlie	Adam

Assuming I want to get only two-person combinations, I would be simply able to do something like that:

SELECT * FROM users u1 
LEFT JOIN users u2 on u1.id!=u2.id

However, I want to include combinations with only one users. How can I get the desired result?

Answer 1

Use a self CROSS JOIN:

SELECT u1.name AS user1,
       NULLIF(u2.name, u1.name) AS user2
FROM users u1 CROSS JOIN users u2
ORDER BY u1.user_id, NULLIF(u2.user_id, u1.user_id);

See the demo.