SQL query for mutual visited places - sql

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

Answer 2

You would have to come up with some parameters for your search. For example, 5 user may have 2 Venues in common, but not 3.
If you want to see what Venues these five users have in common, you can start by doing this:
SELECT a.venue_id, count(1) as NoOfUsers
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
group by a.venue_id
That would bring you, for those users, how many users have that venue. So you have degrees of "Venue sharing".
But if you want to see ONLY the venues who were visited by the five users, you'd add a line in the end:
SELECT a.venue_id, count(1) as NoOfUsers
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
group by a.venue_id
having count(1) = 5 --the number of users in the query
You should also consider changing your WHERE statement from
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
to
WHERE a.user_id in (116, 227, 229, 613, 879)

Answer 3

in sql it would be something like:
Select distinct v.venue_id
from v.venues
join activities a on a.venue_id = v.venue_id
Join users u on u.user_id = a.user_id
Where user_id in (116,227,229,613,879)
You need to join up your tables so to get all the venues that have had activities that have had users. When you are just learning it is sometimes simpler to visualize it if you use subqueries. At leasts thats what I found for me.