Question

Say I have the following table:

create table ratings (
user_id int unsigned not null,
post_id int unsigned not null,
rating set('like','dislike'),
primary key (user_id, post_id)
);

And two users, with ids 1 and 2. I want to find out how much they agree in their ratings, so what I need is a way to select all the posts that both users rated, together with the rating of each user towards each post. Something like:

+---------+---------+---------+
| post_id | rating1 | rating2 |
+---------+---------+---------+
|       1 | like    | dislike |
|       2 | like    | like    |
|       5 | like    | dislike |
|       6 | dislike | dislike |
+---------+---------+---------+

Any clue is appreciated, thanks!

Answer 1

Join your ratings table on itself within post id, and select the rating from each table, restricting the user:

SELECT r1.post_id, r1.rating AS rating1, r2.rating AS rating2
FROM ratings r1
JOIN ratings r2 ON r1.post_id=r2.post_id AND r1.user_id!=r2.user_id
WHERE r1.user_id=1 AND r2.user_id=2