MySQL: Finding users with similar interests - sql

Question

I have 2 tables in my SQL db:
Users: id | email | religion | politics
Interests: id | user_id | interest_name
Given a user1 id, what is the best way to find a second user with at least 1 matching interest? Also, note that the religion/politics in the Users table should also be used for this match.
Any help appreciated,
- Andy

Answer 1

select * from users
where id in (
select id from interests where interest_name in
( select interest_name from interests where id = :current_user_id ))

Answer 2

(PostgreSQL)
I won't argue with your decision to elevate religion and politics above ordinary interests. (That doesn't mean it's a good idea; it just means I won't argue with you about it.)
create table users (
user_id integer primary key,
email_addr varchar(35) not null,
current_religion varchar(35) not null default 'None',
current_politics varchar(35) not null default 'None'
);
insert into users values
(1, 'user#userdomain.com', 'Muslim', 'Liberal'),
(2, 'user#differentdomain.com', 'Muslim', 'Conservative'),
(3, 'user#yadn.com', 'Christian', 'Liberal');
create table interests (
user_id integer not null references users (user_id),
user_interest varchar(20) not null,
primary key (user_id, user_interest));
insert into interests values
(1, 'Walks on the beach'),
(1, 'Women'),
(1, 'Polar bears'),
(2, 'Walks on the beach'),
(2, 'Women'),
(2, 'Little Big Man'),
(3, 'Running on the beach'),
(3, 'Coffee'),
(3, 'Polar bears');
-- Given one user id (1), find a different user with at least
-- one matching interest. You can do this without referring
-- to the users table at all.
select t1.user_id, t1.user_interest, t2.user_id
from interests t1
inner join interests t2 on (t2.user_interest = t1.user_interest)
where t1.user_id = 1 and t2.user_id <> 1;
Returns
1 Walks on the beach 2
1 Women 2
1 Polar bears 3
To also match on, say, religion, you can do essentially the same thing with the table "users".
select t1.user_id, t1.current_religion as interest, t2.user_id
from users t1
inner join users t2 on (t1.current_religion = t2.current_religion)
where t1.user_id = 1 and t2.user_id <> 1
Returns
1 Muslim 2
You can exploit the similar structure to bring religious interests and ordinary interests together using UNION.
select t1.user_id, t1.current_religion as interest, t2.user_id
from users t1
inner join users t2 on (t1.current_religion = t2.current_religion)
where t1.user_id = 1 and t2.user_id <> 1
union
select t1.*, t2.user_id
from interests t1
inner join interests t2 on (t2.user_interest = t1.user_interest)
where t1.user_id = 1 and t2.user_id <> 1;
Returns
1 Walks on the beach 2
1 Women 2
1 Polar bears 3
1 Muslim 2