Question

I’m working on a project for my University with Rails 3/PostgreSQL, where we have Users, Activities and Venues. An user has many activities, and a venue has many activities. An activity belongs to an user and to a venue and has therefore an user_id and a venue_id.

What I need is a SQL query (or even a method from Rails itself?) to find mutual venues between several users. For example, I have 5 users that have visited different venues. And only 2 venues got visited by the 5 users. So I want to retrieve the 2 venues.

I’ve started by retrieving all activities from the 5 users:

SELECT a.user_id as user, a.venue_id as venue
FROM activities AS a
WHERE a.user_id=116 OR a.user_id=227 OR a.user_id=229 OR a.user_id=613 OR a.user_id=879

But now I need a way to find out the mutual venues.
Any idea?

thx,
tux

Answer 1

I’m not entirely familiar with sql syntax for postgresql, but try this:


select venue_id, COUNT(distinct user_id) from activities
Where user_id in (116,227,229,613,879)
group by venue_id
having COUNT(distinct user_id) = 5


EDIT:

You will need to change the ‘5’ to however many users you care about (how many you are looking for).

I tested this on a table structure like so:

user_id     venue_id    id
----------- ----------- -----------
1           1           1
2           6           2
3           3           3
4           4           4
5           5           5
1           2           6
2           2           7
3           2           8
4           2           9
5           2           10

The output was:

venue_id    
----------- -----------
2           5