MySQL - Count number of rows in each group - mysql

Question

I have a SQL table user_game which contains the games that a user owns:
| id | user_id | game_id |
|----|---------|---------|
| 83 | 1 | 1 |
| 84 | 1 | 2 |
| 85 | 1 | 3 |
| 86 | 2 | 2 |
| 87 | 2 | 3 |
| 88 | 2 | 4 |
| 89 | 3 | 2 |
I am trying to count the number of users which have 1 game, 2 games, 3 games.. etc.
User 1 has 3 games, User 2 has 3 games, and User 3 has 1 game. Therefore these are the results I want to achieve:
| no_of_games | COUNT(no_of_games) |
|-------------|--------------------|
| 1 | 1 |
| 2 | 0 |
| 3 | 2 |
COUNT(no_of_games) is the number of users that have that number of games.
I can individually get the number of users for each no_of_games with this query:
-- Select no. of users with 1 game
SELECT no_of_games, COUNT(no_of_games)
FROM
(
-- Select no. of games each user has
SELECT user_id, COUNT(1) as no_of_games
FROM user_game
GROUP BY user_id
) as A
WHERE no_of_games = 1;
which gives the results:
| no_of_games | COUNT(no_of_games) |
|-------------|--------------------|
| 1 | 1 |
However I have to change the no_of_games = 1 to 2, 3, 4... manually and UNION them with this solution and I can't do it for ~60 cases.
Is there a simpler way to achieve this?

Answer 1

Your problem is a bit tricky, because groups of games which do not appear in your data with a certain frequency (e.g. 2) will not appear in the result set just using your original table. In the query below, I use a second table called nums which simply contains the sequence 1 through 10 representing counts of number of games. By using a LEFT JOIN we can retain each game count in the final result set.
SELECT t1.no_of_games,
COALESCE(t2.no_of_games_count, 0) AS no_of_games_count
FROM nums t1
LEFT JOIN
(
SELECT t.no_of_games, COUNT(*) AS no_of_games_count
FROM
(
SELECT COUNT(*) AS no_of_games
FROM user_game
GROUP BY user_id
) t
GROUP BY t.no_of_games
) t2
ON t1.no_of_games = t2.no_of_games
ORDER BY t1.no_of_games
And here is the definition I used for nums:
CREATE TABLE nums (`no_of_games` int);
INSERT INTO nums (`no_of_games`)
VALUES
(1),(2),(3),(4),(5),(6),(7),(8),(9),(10);
Demo here:
SQLFiddle

Answer 2

You can find count of games for each user and then find count of users for each count of games.
select cnt no_of_games, count(*) cnt_no_of_games
from(
select user_id, count(*) cnt
from your_table
group by user_id
) t group by cnt;