Merge based on "group by" groups - mysql

Question

So I have a table called the Activities table that contains a schema of user_id, activity
There is a row for each user, activity combo.
Here is a what it might look like (empty rows added to make things easier to look at, please ignore):
| user_id | activity |
|---------|-----------|
| 1 | swimming | -- We want to match this
| 1 | running | -- person's activities
| | |
| 2 | swimming |
| 2 | running |
| 2 | rowing |
| | |
| 3 | swimming |
| | |
| 4 | skydiving |
| 4 | running |
| 4 | swimming |
I would like to basically find all other users with at least the same activities as a given input id so that I could recommend users with similar activities.
so in the table above, if I wanna find recommended users for user_id=1, the query would return user_id=2 and user_id=4 because they engage in both swimming, running (and more), but not user_id=3 because they only engage in swimming
So a result with a single column of:
| user_id |
|---------|
| 2 |
| 4 |
is what I would ideally be looking for
As far as what I've tried, I am kinda stuck at how to get a solid set of user_id=1's activities to match against. Basically I'm looking for something along the lines of:
SELECT user_id from Activities
GROUP BY user_id
HAVING input_user_activities in user_x_activities
where user1_activities is just a set of our input user's activities. I can create that set using a WITH input_user_activities AS (...) in the beginning, what I'm stuck at is the user_x_activities part
Any thoughts?

Answer 1

To get users with the same activities, you can use a self join. Let me assume that the rows are unique:
select a.user_id
from activities a1 join
activities a
on a1.activity = a.activity and
a1.user_id = #user_id
group by a.user_id
having count(*) = (select count(*) from activities a1 where a1.user_id = #user_id);
The having clause answers your question -- of getting users that have the same activities as a given user.

Answer 2

You can easily get all users ordered by similarity using a JOIN (that finds all common rows) and a GROUP BY (to summarize the similarity per user_id) and finally an ORDER BY to return the most similar users first.
SELECT b.user_id, COUNT(*) similarity
FROM activities a
JOIN activities b
ON a.activity = b.activity
WHERE a.user_id = 1 AND b.user_id != 1
GROUP BY b.user_id
ORDER BY COUNT(*) DESC
An SQLfiddle to test with.