Select users who answered all the questions - mysql

Question

I have tables like these:
Table ANSWER
idAnswer idQuestion Status idGame
----------------------------------------
1 9 1 1
2 6 NULL 1
3 6 1 2
4 3 NULL 2
5 1 1 3
6 6 1 3
7 9 1 4
8 6 1 4
Table GAME:
idGame idUser
----------------
1 Greg
2 Greg
3 Jack
4 Frank
I want to get only those who answered the same question as the game n°4, like this:
Desired result:
idUser
-------
Greg
Frank
Status is used to check if the question has been answered. If it is NULL, the user didn't answer it.
Here, Frank had a very similar game (it's obvious, because it's the same, the n°4).
Greg didn't played a similar game, but answered in 2 others the same questions, so he appears in the result.
Jack did answered one of the question (n°6) but didn't answered the other one (n°9), so Jack doesn't appear.
So, the result includes only those who answered the same questions as another game, no matter if it's in a similar game or in multiple one.
Games are randomly generated. Sometimes, user can answer a question they already encounter or/and answered. (Like Greg, in game 1 and 2).
I tried a lot of queries. I can post them if you want.
Thanks for the answers!

Answer 1

You can try this:
SELECT g.idUser, COUNT(*) as NumberOfAnswers
FROM answer a
INNER JOIN game g ON a.idGame = g.idGame
WHERE a.idQuestion IN (9,6)
GROUP BY g.idUser
HAVING NumberOfAnswers = 2
I think this solution match your requirements if the users cannot answer the same question twice in the same game and if you doesn't mind to show users who has answered more questions.
EDIT:
For a more generic solution, you could adapt the query to something like this:
SELECT g.idUser, COUNT(*) as NumberOfAnswers
FROM answer a
INNER JOIN game g ON a.idGame = g.idGame
WHERE a.idCuestion IN (SELECT idQuestion FROM answer WHERE idGame=[DESIRED_GAME])
GROUP BY g.IdUser
HAVING NumberOfAnswers = (SELECT COUNT(*) FROM answer WHERE idGame = [DESIRED_GAME]