SQL statement I can't wrap my head around (brain too small) - php

Question

I'm writing sort of a travel-'dating' app.
Users register themselves
Users tell the app if they are male or female
Users tell the app which countries they would like to visit
Users tell the app if they want to travel with males (pref_m=1) or females (pref_f=1)
My tables
table 1: users
id (key) | gender | pref_m | pref_f
------------------------------------
1 male 1 0
2 male 1 1
table 2: countryselection
id (key) | userid | countryid
------------------------------------
1 1 123
2 1 111
3 1 100
4 1 110
5 2 123
6 2 111
7 2 202
8 2 210
So what the select statement has to do
Input: the userid of the current user
Output (in logic): SELECT the userids AND matching countries OF ALL people that want to travel to the same countries as I do, and want to travel with someone that has my gender
(join) Of that selection I obviously only need the people that are of the gender that I am looking for.
ORDERED by people that have the most matching countries with me DESC.
What I have so far (warning: not much)
$sql = "SELECT userid,count(*) AS matches from countryselection";
$sql .= " WHERE countryid IN (SELECT countryid FROM countryselection WHERE userid = :userid) GROUP BY userid ORDER BY matches DESC;";
This gives me a list of all people that want to travel to the same countries as me (and how many countries we have in common)
final note
I'm obviously struggling with the gender-selection part.
Not sure if I have done the right thing to store the user selections in the way that I have.
I might need some guidance there too.
Obviously - thanks all.

Answer 1

SELECT
us2.id, -- etc.
COUNT(cs2.countryid) as countries_in_common
FROM
countryselection cs1 -- let's gather user countries he want to visit
LEFT JOIN -- now let's find other users!
countryselection cs2 ON
(
cs2.userid <> :userid AND -- which are not him
cs2.countryid = cs1.countryid -- and want to visit same countries
)
INNER JOIN -- let's grab our user_data
users us1 ON
(
us1.id = cs1.userid
)
INNER JOIN -- and let's grab other user data!
users us2 ON
(
us2.id = cs2.userid
)
WHERE
cs1.userid = :userid AND -- finding our user countries he want to visit
-- final checks
(
(us1.pref_m = 1 AND us2.gender = 'male')
-- he is looking for male and second user is male
OR
(us1.pref_f = 1 AND us2.gender = 'female')
-- he is looking for female and second user is female
) AND
(
(us2.pref_m = 1 AND us1.gender = 'male')
OR
(us2.pref_f = 1 AND us1.gender = 'female')
)
GROUP BY
cs2.userid -- finally group by user_id
Best thing is there are no sub-queries, and you can easily use this query in many ways. (changing order, group by, and using aggregate functions)

Answer 2

It's pretty easy if you don't do the sorting by most countries in common (you could do it in code later if the result sets won't be too large):
SELECT
o.id userid, u_cs.countryid
FROM users u
JOIN countryselection u_cs ON (u.id = u_cs.userid)
JOIN countryselection o_cs ON (u_cs.countryid = o_cs.countryid)
JOIN users o ON (o_cs.userid = o.id)
WHERE
u.id = :userid AND -- The user we want
u.id <> o.id AND -- Exclude ourselves
( -- Check whether the other person is
-- compatible with us
(u.pref_m = 1 AND o.gender = 'male') OR
(u.pref_f = 1 AND o.gender = 'female')
) AND
( -- Check whether we're compatible with the
-- other person
(o.pref_m = 1 AND u.gender = 'male') OR
(o.pref_f = 1 AND u.gender = 'female')
)
SQL Fiddle
If you do want the sorting, I think the best option is to use GROUP_CONCAT (because MySQL sucks and doesn't support windowing/analytic functions).
SELECT
o.id userid, GROUP_CONCAT(u_cs.countryid) countries
FROM users u
JOIN countryselection u_cs ON (u.id = u_cs.userid)
JOIN countryselection o_cs ON (u_cs.countryid = o_cs.countryid)
JOIN users o ON (o_cs.userid = o.id)
WHERE
u.id = :userid AND -- The user we want
u.id <> o.id AND -- Exclude ourselves
( -- Check whether the other person is
-- compatible with us
(u.pref_m = 1 AND o.gender = 'male') OR
(u.pref_f = 1 AND o.gender = 'female')
) AND
( -- Check whether we're compatible with the
-- other person
(o.pref_m = 1 AND u.gender = 'male') OR
(o.pref_f = 1 AND u.gender = 'female')
)
GROUP BY o.id
ORDER BY COUNT(u_cs.countryid) DESC
You could probably pull this off with some nasty subqueries too, but I get the feeling it'll kill performance.
SQL Fiddle

Answer 3

SELECT t4.id, COUNT(t4.id) AS frequency
FROM users t1
LEFT JOIN countryselection t2
ON t1.id = t2.userid
INNER JOIN countryselection t3
ON t2.userid != t3.userid AND t2.countryid = t3.countryid
INNER JOIN users t4
ON t3.userid = t4.id
AND ((t4.pref_m = 1 AND t1.gender = 'male' OR t4.pref_f = 1 AND t1.gender = 'female')
AND (t1.pref_m = 1 AND t4.gender = 'male' OR t1.pref_f = 1 AND t4.gender = 'female'))
WHERE t1.id = ?
GROUP BY t4.id
ORDER BY frequency DESC
Similar to others here but using appropriate join types and join conditions instead of where conditions.
From MySQL docs:
The conditional_expr used with ON is any conditional expression of the
form that can be used in a WHERE clause. Generally, you should use the
ON clause for conditions that specify how to join tables, and the
WHERE clause to restrict which rows you want in the result set.

Answer 4

I think this works
select me.id meid
, them.id themid
, me.gender mygender
, them.gender themgender
, me.pref_m mepref_m
, me.pref_f mepref_f
, them.pref_m thempref_m
, them.pref_f thempref_f
, csme.countryid
from users me
cross
join users them
inner
join countryselection csme
on me.id = csme.userid
inner
join countryselection csthem
on them.id = csthem.userid
where csme.countryid = csthem.countryid
and ((me.gender = 'male' and them.pref_m) or (me.gender = 'female' and them.pref_f))
and ((them.gender = 'male' and me.pref_m) or (them.gender = 'female' and me.pref_f))
and me.id != them.id
and me.id = 2
http://sqlfiddle.com/#!2/06351/25/0
I intentionally left out the group by so that the results are more easily verified.

Answer 5

inferred DDL :
create table users (id int, gender text, pref_m bool, pref_f bool);
create table countryselection (id int, userid int, countryid int);
Here is CSV that you can .import (using sqlite3, after doing .separator ,) into the tables in the question:
users.csv:
1,male,1,0
2,male,1,1
countryselection.csv
1,1,123
2,1,111
3,1,100
4,1,110
5,2,123
6,2,111
7,2,202
8,2,210
peter's sql, edited to use field names from question:
SELECT
us2.id,
COUNT(cs2.*) as countries_in_common
FROM
countryselection cs1
LEFT JOIN
countryselection cs2 ON
(
cs2.userid <> $userid AND
cs2.countryid = cs1.countryid
)
LEFT JOIN
users us1 ON
(
us1.id = cs1.userid
)
LEFT JOIN
users us2 ON
(
us2.id = cs2.userid
)
WHERE
cs1.userid = $userid AND
cs2.userid IS NOT NULL AND
(
(us1.pref_m = 1 AND us2.gender = 'male')
OR
(us1.pref_f = 1 AND us2.gender = 'female')
) AND
(
(us2.pref_m = 1 AND us1.gender = 'male')
OR
(us2.pref_f = 1 AND us1.gender = 'female')
)
GROUP BY
cs2.userid
;
you can execute it like this:
sqlite3 myDBname < peters_sql.sql
setting $userid to 1, i get 2 as output.

Answer 6

UPDATE: (added the countries)
SELECT u1.id AS uid1
, u2.id AS uid2
, cs.countryid
FROM users u1
, users u2
JOIN countryselection cs ON cs.userid = u2.id
-- WHERE u1.id < u2.id -- tiebreaker
WHERE u1.id = 12345
AND EXISTS (
SELECT * FROM countryselection cs1
JOIN countryselection cs2 ON cs1.countryid = cs2.countryid
WHERE cs1.userid = u1.id
AND cs2.userid = u2.id
)
AND ((u1.pref_m = True AND u2.gender = 'male')
OR (u1.pref_f = True AND u2.gender = 'female') )
-- the love must be mutual ...
AND ((u2.pref_m = True AND u1.gender = 'male')
OR (u2.pref_f = True AND u1.gender = 'female') )
;