Question

Table structure:

uid       : integer
answer_id : integer

I need to run a query which will show me which uid’s have the same answers as other uid’s. So for example, here’s some test data:

answer_id   uid
1           555
4           555
7           555
10          555
1           123
5           123
7           123
10          123

So we can see from this data that they each answered 3/4 of the questions the same way.

I’m struggling with how to write a query which would show me which uid’s match 3/4 or 4/4 of the same answers. Basically I’m trying to find users with 75% (3/4) or greater (4/4) similar answers.

This is part of a Ruby on Rails application, so I have the models all built [User, UserAnswers etc..] but I’m assuming this will just be a SQL query, not necessarily a part of ActiveRecord

Answer 1

This query show the number of answers every user has in common with each other:

declare @uid int

select
  ans1.uid as user1,
  ans2.uid as user2,
  count(*)
from 
  ans ans1 inner join ans ans2
  on ans1.answer_id = ans2.answer_id
     and ans1.uid <> ans2.uid
where uid = @uid
group by user1, user2
having count(*)>0

This also shows the number of questions each user has answered:

 select
  ans1.uid as user1,
  ans2.uid as user2,
  count(distinct ans1.answer_id) as total1,
  count(distinct ans2.answer_id) as total2,
  sum(case when ans1.answer_id = ans2.answer_id then 1 else 0 end) as common
from 
  ans ans1 inner join ans ans2 on ans1.uid <> ans2.uid
group by user1, user2
having count(*)>0

(this second query can be very slow)