SQL - complicate JOIN query - php

Question

In a tutorial I saw this SQL query:
SELECT a.id as a_id, a.info as a_info,
b.id as b_id, b.info as b_info
FROM stats AS a
JOIN stats as b
ON a.id != b.id
LIMIT 25
The query is working well, but now I would need to add the WHERE condition. I have the table users. The table stats contains the column user_id.
I try to obtain all users from the table stats if users.city=1 (for example), but I still cannot to find a way, how to achieve that...
My problem is, that I currently have no clue, how to put into the query another JOIN (for table users).
I would be grateful for every advice, thank you

Answer 1

Well, as you use the table stats two times in your query, you would actually need two joins for the users table as well. (for stats A and stats B)
SELECT a.id as a_id, a.info as a_info,
b.id as b_id, b.info as b_info
FROM stats AS a
JOIN stats AS b ON a.id != b.id
JOIN users AS a_users ON a.user_id = a_users.id
JOIN users AS b_users ON b.user_id = b_users.id
WHERE a_users.city = 1
OR b_users.city = 1
LIMIT 25

Answer 2

So this is what you've got...
This is what you return
SELECT a.id as a_id, a.info as a_info, b.id as b_id, b.info as b_info
This is where it comes from
FROM stats AS a
JOIN stats as b on a.id != b.id
This is how many you want
LIMIT 25
So if you want to add the users table, add this after the other joins
JOIN users ON a.user_id = users.id
and to filter it, add this after the joins
WHERE users.city=1
Giving
SELECT a.id as a_id, a.info as a_info, b.id as b_id, b.info as b_info
FROM stats AS a
JOIN stats as b on a.id != b.id
JOIN users ON a.user_id = users.id
WHERE users.city=1
LIMIT 25

Answer 3

Here is one way to do that:
SELECT a.id as a_id, a.info as a_info,
b.id as b_id, b.info as b_info
FROM stats AS a
JOIN stats as b
ON a.id != b.id
where a.user_id in (select user_id from users where users.city = 1)
LIMIT 25
You can also express this as a join (which in MySQL is more efficient):
SELECT a.id as a_id, a.info as a_info,
b.id as b_id, b.info as b_info
FROM stats AS a
JOIN stats AS b
ON a.id != b.id
JOIN users AS u
ON a.user_id = u.user_id
AND u.city_id = 1
LIMIT 25

Answer 4

For convenience, I've reformatted your original SQL statement to make it readable by humans.
SELECT a.id AS a_id
, a.info AS a_info
, b.id AS b_id
, b.info AS b_info
FROM stats a
JOIN stats b
ON a.id != b.id
LIMIT 25
Your current query is almost returning a Cartesian product. Every row from stats is getting matched with every row from stats, except for matching itself. (Assuming that stats.id is a primary key or unique key.)
To add a join to the users table, to limit rows returned from a, for example:
SELECT a.id AS a_id
, a.info AS a_info
, b.id AS b_id
, b.info AS b_info
FROM stats a
JOIN users au ON au.id = a.user_id AND au.city=1
JOIN stats b ON a.id != b.id
LIMIT 25
If you want to limit rows returned for both a and b, add another join to the users table:
SELECT a.id AS a_id
, a.info AS a_info
, b.id AS b_id
, b.info AS b_info
FROM stats a
JOIN users au ON au.id = a.user_id AND au.city=1
JOIN stats b ON a.id != b.id
JOIN users bu ON bu.id = b.user_id AND bu.city=1
LIMIT 25
This is not the only way to accomplish this. For example, you could use an a.user_id IN (subquery) or an EXISTS (subquery) predicate.
(SQL is much easier to work with if you have it formatted in a way that is readable.)