Query to get the common values - mysql

Question

I have a table like
userId classId
1 1
2 1
3 1
1 2
2 2
1 3
2 4
3 5
In the above, [1, 2, 3] share class 1. [1, 2] share class 2, and so on.
If I give userId 1, 2, 3 in a query that should return classId 1.
If I give 1 and 2, the query should return 2.
If I give 1, the query should return 3.
How can I write a MySQL query for that?

Answer 1

The relational operator you require is division, popularly known as "the supplier who supplies all parts".
Because you don't consider class = 1 to be a valid result when given user = {1, 2}, you should be looking at exact division (i.e. without a remainder). You also need to consider what the result should be given an empty divisor.

Answer 2

This was tested with Postgresql, but I'm pretty sure it should work without much modifications under mysql.
Table setup
drop table classes;
create table classes (
user_id integer,
class_id integer
);
insert into classes values(1, 1);
insert into classes values(2, 1);
insert into classes values(3, 1);
insert into classes values(1, 2);
insert into classes values(2, 2);
insert into classes values(1, 3);
insert into classes values(2, 4);
insert into classes values(3, 5);
1, 2, 3
select max(class_id) from
(
select class_id
from classes
where user_id = 1
intersect
select class_id
from classes
where user_id = 2
intersect
select class_id
from classes
where user_id = 3
) as foo;
1, 2
select max(class_id) from
(
select class_id
from classes
where user_id = 1
intersect
select class_id
from classes
where user_id = 2
) as foo;
1 (Could be simplified, but I prefer it so, for symmetry reasons)
select max(class_id) from
(
select class_id
from classes
where user_id = 1
) as foo;

Answer 3

You could use a query like this (for users 1,2,3) -
SELECT t1.classId FROM (SELECT * FROM users_classes WHERE userId = 1) t1 -- for userId = 1
JOIN (SELECT * FROM users_classes WHERE userId = 2) t2 ON t1.classId = t2.classId -- for userId = 2
JOIN (SELECT * FROM users_classes WHERE userId = 3) t3 ON t1.classId = t3.classId -- for userId = 3
For users 1,2 -
SELECT t1.classId FROM (SELECT * FROM users_classes WHERE userId = 1) t1 -- for userId = 1
JOIN (SELECT * FROM users_classes WHERE userId = 2) t2 ON t1.classId = t2.classId -- for userId = 2
These query will output all possible classId, to get one classId you can add LIMIT or aggregate function.

Answer 4

Try this
SELECT DISTINCT (
classId
)
FROM `data`
WHERE userid =1
OR userid =2 or classid = 3
LIMIT 0 , 1
SELECT DISTINCT (
classId
)
FROM `data`
WHERE userid =1
OR userid =2
LIMIT 1 , 1
SELECT DISTINCT (
classId
)
FROM `data`
WHERE userid =1
LIMIT 2 , 1