SQL query to get at least one common item - sql

Question

I have 3 tables which are,
room (room_no)
room_no_access (room_no, user_id)
user (user_id)
If a user is recorded in room_no_access table with a room_no and user_id then that user cannot access the mentioned room.
Let's assume that I am a user in user table. Now what I need is to get the users who shares at least one common room access with me.
How to write a SQL to get this done? It's better if there are no loops in it.
Example:
rooms
room_1,
room_2,
room_3,
users
1,
2,
3,
4,
rooms_no_access
[room_1, 4],
[room_2, 3].
[room_3, 3],
[room_1, 3],
If I'm user 4 -> I have access to room_2, room_3
So who are the other users who has access to room_2 or room_3
They are ->1,2

Answer 1

This is kind of back to front, as you hold which rooms people DON'T have access to, and we really want rooms people DO have access to.
I created some test data:
--Test data
DECLARE #room TABLE (room_no INT);
INSERT INTO #room SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3;
DECLARE #user TABLE ([user_id] INT);
INSERT INTO #user SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3;
DECLARE #room_no_access TABLE (room_no INT, [user_id] INT);
INSERT INTO #room_no_access SELECT 1, 1;
INSERT INTO #room_no_access SELECT 3, 2;
INSERT INTO #room_no_access SELECT 1, 3;
INSERT INTO #room_no_access SELECT 2, 3;
INSERT INTO #room_no_access SELECT 3, 3;
...and then came up with this query:
--For each user find users with common access
--First find which rooms each user can access
WITH room_access AS (
SELECT
r.room_no,
u.[user_id]
FROM
#room r
CROSS JOIN #user u
LEFT JOIN #room_no_access rn ON rn.room_no = r.room_no AND rn.[user_id] = u.[user_id]
WHERE
rn.[user_id] IS NULL)
SELECT DISTINCT
u.[user_id],
sa.[user_id] AS shared_access_user
FROM
#user u
LEFT JOIN room_access ra ON ra.[user_id] = u.[user_id]
LEFT JOIN room_access sa ON sa.room_no = ra.room_no
WHERE
sa.[user_id] != u.[user_id]; --Ignore self match
That's a little long-winded for the job, but it's easy to follow.
Results are:
user_id shared_access_user
1 2
2 1
Because in my data user_id #1 and #2 share access to room #2, but user_id #3 has no access to any rooms.
I am returning the results for each user in the database, but you could filter this to only return results for a specified user?

Answer 2

Try this:
select distinct [user_id] from (
--all rooms that can be accessed by at least one user
select room_no, [USER_ID] from room cross join [user]
except
select room_no, [USER_ID] from room_no_access
) a where room_no in (
-- rooms that I have access to
select room_no from room
except
select room_no from room_no_access where [USER_ID] = /*my_user_id*/
)

Answer 3

This select users that share access with you and the rooms they shares.
set #you = 1;
select
others_access.user_id,
others_access.room_no
from
(select room_no, #you as user_id
from room r
/* rooms that you have access */
where not exists (
select * from room_no_access rna
where rna.room_no = r.room_no
and rna.user_id = #you)) you_access
join
/* rooms that others have access */
(select room_no, u.user_id
from room r cross join user u
where u.user_id <> #you
and not exists (
select * from room_no_access rna
where rna.room_no = r.room_no
and rna.user_id = u.user_id)) others_access
on you_access.room_no = others_access.room_no;
Here a DB Fiddle to play with