Question

Say I have three tables that look like:

users:
id

event_registration:
id
user_id
event_id

events:
id

A user registers for an event and this registration is recorded in event_registration.

Is there a way I can calculate the count of events that a given set of users mutually share in a single MySQL statement? I figured out how to do this for two users, but I’d like to refactor
it to support X number of users (including just one user).

Answer 1

try

select count(event_id) as events
from event_registration
where user_id in (1,2,3)
group by event_id
having count(distinct user_id) = 3

You have to adjust the number 3 in the having clause acourding to the number of user_ids you are using in your in clause.